Fiber-Optic Chemical Sensors and Biosensors

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Anal. Chem. 2008, 80, 4269–4283

Fiber-Optic Chemical Sensors and Biosensors
Otto S. Wolfbeis

Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany

Review Contents available now with transmissions over a wide spectral range.
Books and Reviews 4269 Current limitations are not so much in the transmissivity but in
Sensors for Gases, Vapors, and Humidity 4271 the (usually shortwave) background ﬂuorescence of most of the
Hydrogen 4271 materials ﬁbers are made from, in particular plastic. There is an
Hydrocarbons 4271 obvious trend toward longwave sensing where background signals
Oxygen 4272
are weaker. Major ﬁelds of applications are in sensing gases and
Other Gases 4272
Vapors 4274 vapors, in medical and chemical analysis, molecular biotechnology,
Humidity 4274 marine and environmental analysis, industrial production monitor-
Sensors for pH and Ions 4275 ing, bioprocess control, and the automotive industry. Note: In this
Sensors for Organic Chemicals 4276
article, sensing refers to a continuous process, while probing refers
Organics 4276
Biosensors 4276 to single-shot testing. Both have their ﬁelds of applications.
Enzymatic Biosensors 4276 FOS are based on either direct or indirect (indicator-based)
Immunosensors 4277 sensing schemes. In the ﬁrst, the intrinsic optical properties of
DNA Biosensors 4278
the analyte are measured, for example its refractive index,
Bacterial Biosensors 4278
Applications 4279 absorption, or emission. In the second, the color or ﬂuorescence
Sensing Schemes and Spectroscopies 4279 of an immobilized indicator, label, or any other optically detectable
Fiber Optics 4279 bioprobe is monitored. Aside from the design of label and probes,
Capillary Waveguides 4280 active areas of research include advanced methods of interrogation
Microsystems and Microstructures 4280
Refractive Index 4280 such as time-resolved or spatially-resolved spectroscopy, evanes-
Spectroscopies 4280 cent wave and laser-assisted spectroscopy, surface plasmon
Literature Cited 4281 resonance (SPR), and multidimensional data acquisition. In recent
years, ﬁber bundles also have been employed for purposes of
This review covers the time period from January 2006 to imaging, for biosensor arrays (along with encoding), or as arrays
January 2008 and is written in continuation of previous reviews (1–3). of nonspeciﬁc sensors whose individual signals may be processed
Data were electronically searched in SciFinder and MedLine. via artiﬁcial neural networks. The success of SPR in general, and
Additionally, references from (sensor) journals were collected by in the form of FOS in particular, is impressive.
the author over the past 2 years. The number of citations in this Following the recent sensor hype of (mainly organic) chemists
review is limited, and a stringent selection had to be made that tend to refer to optical molecular probes as “sensors”, the
therefore. Priority was given to ﬁber-optic sensors (FOS) of literature has become more difﬁcult to sort. This review covers
deﬁned chemical, environmental, or biochemical signiﬁcance and literature on methods that enable sensing of (bio)chemical species
to new schemes. as opposed to conventional types of optical assays. Unfortunately,
The review does not include the following: (a) FOS that there is a tendency to even refer to conventional indicators (such
obviously have been rediscovered; (b) FOS for nonchemical as for pH or calcium) as biosensors if only used in vivo. Similarly,
species such as temperature, current and voltage, stress, strain, optical analysis of a solution by adding an appropriate indicator
displacement, structural integrity (e.g., of constructions), liquid probe is now referred to as “sensing” (to the surprise of the sensor
level, and radiation; and (c) FOS for monitoring purely technical community). I have outlined the situation in more detail in my
processes such as injection molding, extrusion, or oil drilling, even previous review (3).
though these are important applications of optical ﬁber technology.
Unfortunately, certain journals publish articles that represent BOOKS AND REVIEWS
marginal modiﬁcations of prior art, and it is mentioned here It is obvious that ﬁber optic chemical sensors (FOCS) have
explicitly that the (nonpeer-reviewed) Proceedings of the SPIE are had a particular success in areas related to sensing gases and
particularly uncritical in that respect. vapors. Many of the systems implemented are based on direct
Fiber optics serve analytical sciences in several ways. First, spectroscopies that range from UV to IR, and from absorbance
they enable optical spectroscopy to be performed at sites inac- to ﬂuorescence and surface plasmon resonance. Optical chemical
cessible to conventional spectroscopy, over large distances, or sensors have been comprehensively reviewed by McDonagh et
even at several spots along the ﬁber. Second, in being optical al. quite recently (4). The article covers sensing platforms, direct
waveguides, ﬁber optics enable less common methods of inter- (spectroscopic) sensors, reagent-mediated sensors and discusses
rogation, in particular evanescent wave spectroscopy. Fibers are trends and future perspectives. Fiber-optic UV systems for gas
10.1021/ac800473b CCC: $40.75  2008 American Chemical Society Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4269
Published on Web 05/08/2008
and vapor analysis have been reviewed (5). The strong absorbance spectro-electrochemical sensing does not relate to electrolumi-
of vapors and gases in the UV region is advantageous and resulted nescence but to electrochemical conversion of an analyte prior to
in a compact detection system of good accuracy. Buchanan has its spectroscopic detection, e.g., by preconcentration, oxidation,
reviewed recent advances in the use of near-IR (NIR) spectroscopy reduction, complexation, and optical detection. Given its complex-
in the petrochemical industry (6) and points out NIR is particularly ity, it does not come as a surprise that the authors point to “the
attractive in this ﬁeld because it measures the overtone and need for further reﬁnement of the sensor’s design and the
combination bands predominately of the C-H stretches. Practical experimental protocol to improve the method’s sensitivity.”
examples include sensing schemes for oxygenates in fuels, The performances of interferometry, surface plasmon resonance,
determination of octane numbers, the composition of fuels, and luminescence as biosensors and chemosensors have been
bitumen analysis, and environmental analysis. Tools for life science critically compared (16). Sensitivity, dynamic range, and resolution
research based on ﬁber-optic-linked Raman and resonance Raman were calculated and compared from a range of data from the
spectroscopy were also reviewed (7). One focus is on ﬁber-optic literature. Theoretical sensitivities of interferometry and SPR are
probes for UV resonance Raman spectroscopy that offer several detailed along with parameters affecting these sensitivities.
advantages over conventional excitation/collection methods, an- Luminescence is said to offer the best resolutions for sensing of
other on novel probes based on hollow-core photonic band gap protein and DNA, while interferometry is said to be most suitable
ﬁbers that virtually eliminate the generation of silica Raman for low-molecular weight chemical liquids and vapors if selectivity
scattering within the excitation optical ﬁber. FOCS for volatile is not a critical issue. SPR (which is label-free) possibly can sense
organic compounds have been reviewed by Elosua et al. (8). Such proteins with a resolution similar to that of luminescence.
sensors are minimally invasive, lightweight, passive and can be Borisov and Wolfbeis (17) have presented what appears to be
multiplexed. The devices were classiﬁed according to their the most comprehensive review on optical biosensors so far.
mechanism of operation and in terms of sensing materials. The Applications of optical ﬁber (bio)sensors (FOBS) also include
state of the art in leak detection and localization and respective areas such as high-throughput screening of drugs (18), the
legal regulations have been reviewed by Geiger (9). Speciﬁc detection of food-borne pathogens (19) (based on either SPR,
aspects include sensor reliability, sensitivity, accuracy, and robust- resonant mirrors, ﬁber-optic systems, arrays, Raman spectroscopy,
ness. Applicability is demonstrated for two examples, a liquid and light-addressable potentiometry), and in environmental sens-
multibatch pipeline and a gas pipeline. Nanostructure-based optical ing by making use of DNAzymes (20). DNAzymes can selectively
ﬁber sensor systems and examples of their application have been identify charged organic and inorganic compounds at ultratrace
reviewed by Willsch et al. (10). Selected examples of advanced levels in waste and emissions. In combination with laser based
optical ﬁber sensor systems based on subwavelength structured detection, DNAzymes enable accurate quantiﬁcation of such
components are presented. These include sensor for humidity and compounds and thus represent an attractive alternative to state-
hydrocarbons, for application in the gas industry, and for envi- of-the-art afﬁnity sensors. Signiﬁcant strides have been made in
ronmental monitoring using nanoporous thin-ﬁlm Fabry-Perot terms of selectivity, sensitivity, and catalytic rates of DNAzymes.
transducer elements or intrinsic ﬁber Bragg grating sensor Challenges remain in the development of efﬁcient signal trans-
networks for structural health monitoring. Additionally, new duction technology for in situ applications. The state of the art in
concepts for sensing based on the use of plasmonic metal continuous glucose sensing, a kind of holy grail in FOBS, has
nanoparticles, photonic crystal ﬁbers, and optical nanowires are been summarized in a book (21), and (ﬁber) optical methods
discussed. based on the use of glucose oxidase and transduction via oxygen
Mohr (11) has reviewed recent developments in chromogenic consumption are reviewed in one chapter (22).
and ﬂuorogenic reagents and sensors for neutral and ionic analytes The trend toward miniaturization is obvious. Nanoscale optical
based on covalent bond formation. New indicator dyes for amines biosensors and biochips for cellular diagnostics have been
and diamines, amino acids, cyanide, formaldehyde, hydrogen reviewed by Cullum (23), speciﬁcally with respect to achievements
peroxide, organophosphates, nitrogen oxide and nitrite, peptides in employing nanosensors and biochips (e.g., gene chips) in
and proteins, as well as for saccharides also are described, and cellular analyses ranging from medical diagnostics to genomics,
new means (such as color changes of chiral nematic layers) of while optical nanobiosensors and nanoprobes were reviewed by
converting analyte recognition into optical signals are described. Vo-Dinh (24) with respect to the principles, development, and
Aspects of multiple optical chemical sensing with respect to applications of ﬁber-optic nanobiosensor systems using antibody-
parameters, materials, and spectroscopies have been reviewe (12). based probes. One speciﬁc class of FOBS, the tapered ﬁber-optic
Borisov et al. summarize their research on plastic microparticles biosensors (TFOBS) were reviewed (25). In these, part of the
and nanoparticles for ﬂuorescent chemical sensing and encoding ﬁber is tapered so that the evanescent ﬁeld of the lightwave can
(13). A rather wide and shallow review covers advances in ﬁber- interact with samples. TFOBS are often used with transduction
optic sensing in medicine and biology (14). Applications range mechanisms such as changes in refractive index, absorption,
from the use of ﬁbers acting as plain light pipes to complex ﬂuorescence, and SPR. A more general review covers recent
chemical sensors. It is stated (somewhat overoptimistically) that developments in FOBS (26), whereas Walt (27) describes ﬁber-
chemical sensing can “simply” be achieved by transporting light optic biosensor arrays for creating high-density sensing arrays.
to and from a measurement site with a plain ﬁber light guide for Femtoliter wells can be loaded with individual beads to create
spectrophotometry, ﬂuorometry, or SPR. Flowers et al. discussed such arrays for multiplexed screening and bioanalysis. Adherent
aspects of ﬁber-optic spectro-electrochemical sensing for in situ cells may be attached to the ﬁber substrate to provide a method
determination of metal ions, mainly copper(II) (15). The term for observing cell migration and for screening antimigratory
4270 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
compounds, and even individual enzyme molecules can be loaded composites of the sol-gel type and used in a reversible hydrogen
into the wells, thus enabling single molecule detection via enzyme- FOCS (37).
catalyzed signal ampliﬁcation. Optical microarray biosensing The gasochromic properties of nanostructured tungsten oxide
techniques (28), in turn, provide a powerful tool for the simulta- ﬁlms coated with a palladium catalyst were used to sense
neous analyses of thousands of parameters, be they DNA or hydrogen gas via the change in the optical transmittance at 645
proteins. The review highlights promising microarray techniques nm, typically caused by 1% hydrogen in argon gas (38). The
either making use of labels or label-free. Rather than miniaturizing nanomorphology of the surface considerably improves the gaso-
the optical ﬁber, these may be covered with nanostructured chromic properties. Two rather similar articles have been pub-
coatings (29). Active and passive coatings, deposited via the lished by this group (39, 40).
Langmuir-Blodgett and electrostatic self-assembly techniques, Luna-Moreno et al. have reported on the effect of hydrogen
may be utilized to affect the transmission of optical ﬁbers. While on a thick ﬁlm Pd-Au alloy (41). The resulting sensor consists
such sensor elements are mainly aimed for use in telecommunica- of a multimode ﬁber in which a short section of single mode ﬁber
tions systems, it is very likely that chemical sensor and biosensor is coated with the Pd-Au ﬁlm. If exposed to hydrogen, the
development may beneﬁt from such research. refractive index of the Pd-Au layer becomes smaller and causes
Jeronimo et al. (30) review optical sensors and biosensors attenuation on the transmitted light. The Pd-Au ﬁlm can detect
based on sol-gel ﬁlms. Applications include sensors for pH, gases, 4% hydrogen with a response time of 15 s. The same material
ionic species and solvents, as well as biosensors. The use of FOCS was used to design a hydrogen sensor based on core diameter
for on-site monitoring and analysis of industrial pollutants with mismatch and annealed Pd-Au thin ﬁlms (42).
respect to detecting the identity, concentrations, and extent of Palladium-capped magnesium hydride was used as an alterna-
toxic chemical contamination was overviewed (31). Aspects of tive sensing material in a ﬁber-optic hydrogen detector (43). A
process monitoring of ﬁber reinforced composites using optical drop in the reﬂectance of this material by a factor of 10 is
ﬁber sensors were summarized (32), with a focus on thermoset- demonstrated at hydrogen levels as low as 15% of the lower
ting resins and on spectroscopy-based techniques that can be used explosion limit. The response occurs within a few seconds.
to monitor the processing of these materials Comparing Mg-Ni and Mg-Ti based alloys, the latter has
A classroom demonstration was described for a portable ﬁber- superior optical and switching properties. A gasochromic TiO2-
optic probe multichannel spectrophotometer (33). With the use based sensing ﬁlm was used for hydrogen detection by means of
of this instrument, lecture demonstrations can be made of various a ﬁber-optic Fabry-Perot interferometric sensor (44). It was
concepts in molecular ﬂuorescence spectroscopy. Concepts in- applied to monitor hydrogen gas in air below the lower explosion
clude ﬂuorescence spectrophotometer design geometry and the limit, has a short response time, and regenerates quickly (at room
correlation of color with emission wavelength, excitation, and temperature).
emission spectra. The history of research on FOCS and FOBS Fiber gratings coated with Pd metal were reported to enable
has been summarized (34). sensing of hydrogen gas (45). Fiber Bragg gratings (FBG) and
long period gratings (LPG), both coated with Pd nanolayers, were
SENSORS FOR GASES, VAPORS, AND HUMIDITY investigated. The sensitivity of the LPG sensor is better by a factor
This section covers all room-temperature gaseous species of ∼500. The FBG sensors appear to be pure strain sensors, and
including their solutions in liquids. One major research focus is LPG sensors are mainly based on the coupling between the
on hydrogen and methane because both are highly explosive when cladding modes and evanescent or surface plasmon waves. In
mixed with air and may be sensed more safely with FOS than another type of FBG sensor, a 10 ppm sensitivity for hydrogen
with electrical devices. was reported (along with cross sensitivity to environmental
Hydrogen. Hydrogen, along with ﬂammable alkanes, remains conditions) (46). The sensor was used to monitor the aging of
to be the analyte where safety considerations have led to a certain materials. Optical ﬁbers coated with single-walled carbon
substantial amount of research in terms of ﬁber-optic sensing. nanotubes (SWCNTs) were shown to enable determination of
Since hydrogen gas does not display intrinsic absorptions/ hydrogen at cryogenic temperatures (47). SWCNTs were depos-
emissions that could be used for purposes of simple optical ited by the Langmuir-Blodgett technique at the distal end of the
sensing, it is always detected indirectly. Hydrogen interacts ﬁbers. Experiments carried out at 113 K revealed the potential of
strongly with metallic palladium and platinum ﬁlms and with sensing <5% of gaseous hydrogen with good reversibility and fast
tungsten oxide. The interactions result in both spectral changes response time.
(an effect sometimes called gasochromism) and in an expansion Hydrocarbons. Methane and other hydrocarbons are almost
of the materials. Thus, a hydrogen sensor was reported based on exclusively sensed via their intrinsic absorption in the near-infrared
palladium coated side-polished single mode ﬁber (35). When (NIR). These, in contrast to hydrogen or oxygen, can be detected
exposed to 4% hydrogen gas, the optimal change output power via their intrinsic absorption in the (near) infrared-albeit cross
obtained in this experiment was 1.2 dB (32%) with a risetime of sections (molar absorbances) remains small or via Raman scatter.
100 s. Improved response times were reported for a similar sensor One more ﬁber-optic Raman sensor was reported for sensing
based on the same scheme (36). The authors have studied the ethanol and methanol in gasoline (48). It employs a frequency
transmission, the time-response, and the initial response velocity doubled 532 nm Nd:YAG laser and a specially designed ﬁber-optic
in the range from -30 to 80 °C. Heating the palladium layer with Raman probe and enables online determination of sample con-
an auxiliary laser diode improves the response time at low stituents without employing an expensive IR ﬁber. The sensor is
temperatures. Palladium also was incorporated into silica nano- capable of monitoring methanol and ethanol in water and gasoline
Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4271
solutions. Surface-enhanced Raman spectroscopy and surface- quenching of the long-lived delayed ﬂuorescence of fullerene C70
enhanced IR spectroscopy were applied to selective determination incorporated in thin ﬁlms of ethyl cellulose or organosilica (59).
of polycyclic aromatic hydrocarbons (49). Molecular recognition Molecular oxygen in concentrations as low as 200-800 ng/L can
is accomplished by functionalization of the silver nanoparticles be determined by this method either at single sensor spots or
with appropriate host molecules. spatially resolved over a temperature range of more than 100 °C.
A modular, mid-IR, evanescent wave ﬁber-optic sensor for the The group of Klimant (60) has presented magnetically separable
detection of hydrocarbon pollutants in water was constructed and optical sensor beads for oxygen. The beads contain Fe3O4 and
tested (50). The setup uses a broadband light source with can be magnetically ﬁxed at the bottom of a microbioreactor and
backreﬂecting optics coupled to a ﬁber-optic sensing element enable contactless monitoring of oxygen in cultures of Escherichia
coated with an analyte-enriching polymer that preconcentrates the coli using ﬁber optics. The same group has presented new and
analyte. Benzene was quantiﬁed down to 500 ppm using a PVC ultrabright ﬂuorescent probes for oxygen that are based on
polymer coating. Three kinds of microstructure ﬁbers (MSFs) cyclometalated iridium(III) coumarin complexes (61). The probes
for sensing gaseous hydrocarbons were reported (51) that enable are less cross-sensitive to temperature, have lifetimes on the order
the quantitation of aromatic hydrocarbons. Their surfaces were of 8-13 µs in the unquenched state, have quantum yields between
modiﬁed by xerogel layers. The MSFs with 10-50 µm air holes 0.3 and 1.0, but are less photostable than ruthenium-based probes
were arranged to one sensor. Capillary silica ﬁbers were also for oxygen.
fabricated. The response of the sensors to toluene in nitrogen Fiber optic microsensors with a tip diameter of ∼140 µm were
gas results from spectral changes of the output light from the reported for simultaneous sensing of oxygen and pH and of
ﬁbers at 1600-1800 nm. A detection limit of 0.007 vol.% of toluene oxygen and temperature (62). The tip of the ﬁber was covered
was achieved. with sensor chemistries based on luminescent microbeads that
Oxygen. Oxygen sensing remains another area where FOCS respond to the respective parameters by a change in the decay
are quite successful. Oxygen almost exclusively is sensed by virtue time, or the intensity of their luminescence, or both. The use of
of the quenching effect it exerts on certain ﬂuorophores. High- microbeads enables the ratio of the signals to be easily varied,
performance ﬁber-optic oxygen sensors based on ﬂuorinated reduces the risk of ﬂuorescence energy transfer between indicator
xerogels doped with quenchable Pt(II) complexes were reported dyes, and reduces the adverse effect of singlet oxygen that is
by Chu et al. (52). The sensors yield linear Stern-Volmer plots, produced in the oxygen-sensitive beads. Arain et al. (63) have
and response times range from 4 to 7 s. Rather similar materials characterized microtiterplates (MTPs) with integrated optical
resulted in even faster responses as reported in a second paper sensors for oxygen and pH and have applied them to enzyme
by this group (53). Most oxygen sensors display nonlinear activity screening, respirometry, and toxicological assays. Thin
Stern-Volmer relationship, and this can be described by the so- hydrophilic sensing ﬁlms consisting of an analyte-sensitive indica-
called two-site model assuming two quenching constants. It has tor and a reference ﬂuorophore were deposited on the bottom of
been shown that an artiﬁcial network also may be applied to model the MTPs. Activity screening was demonstrated for glucose
the dynamic quenching of the ﬂuorescence of a ruthenium-derived oxidase and for monitoring the growth of E. coli and Pseudomonas
probe (54). putida. A toxicological assay also is reported that enables monitor-
A porous plastic sensor was developed for the determination ing the respiratory activity of P. putida. In order to compensate
of dissolved oxygen in seawater (55). The luminescent indicator, for the rather strong effects of temperature on oxygen sensors
a ruthenium(II)diimine complex, was copolymerized with a based on dynamic quenching of luminescence, Borisov et al. (64)
polymerizable monomer, a cross-linking reagent, and a porogen. have developed a composite luminescent material for dual sensing
It did not leach out due to its high hydrophobicity. Sensing is of oxygen and temperature, where temperature can be measured
based on dynamic quenching of the ﬂuorescence of the ruthenium optically and used to calculate temperature-corrected data for pO2.
indicator. The permeability of contact lenses for oxygen can be Quantum dots undergo temperature-dependent changes of the
determined with a ﬁber-optic luminescent sensor system (56). The intensity, the peak wavelength, and the spectral width of their
method is based on kinetic measurements of the oxygen partial luminescence. Hence, they can be used as probes to compensate
pressure inside a chamber sealed by the sample contact lens, for effects of temperature in FOCS (65). Effects are almost linear
where a thin luminescent O2-sensitive ﬁlm is being placed. Unlike and fully reversible. A resolution of 0.3 °C was achieved.
in electrochemical techniques, the optical sensor is unaffected by Oxygen and carbon dioxide are clinically highly signiﬁcant
the thickness of the contact lens and other effects. blood gases. A composite ﬂuorescent material was described that
Oxygen sensors were used to transduce the activity of catalase enables simultaneous sensing and imaging of oxygen and carbon
(57). This is of interest to characterize the oxidative metabolism dioxide (66). It relies on the measurement of the phase shift of
in coffee cherries during maturation as it appears to be regulated the luminescence decay time of a material composed of microbead-
by the timely expression of redox enzymes such as catalase (CAT), contained indicators (with well-separated excitation and emission
peroxidase, and polyphenoloxidase. The assay allows for the wavelengths) and polymers with excellent permeation selectivities
screening of green coffee samples for CAT activities. An evanes- as well as favorable optical and adhesive properties. A luminescent
cent wave ﬁber sensor was used to determine oxygen deﬁciency dual sensor for time-resolved imaging of pCO2 and pO2 in aquatic
(58). The sensing dye, methylene blue, was immobilized in the systems was reported by Schroeder et al. (67).
cladding using a sol-gel process. The sensor is said to respond Other Gases. Ozone gas was sensed via its UV and visible
logarithmically linear between 0.6% and 20.9% oxygen. The most absorption (68). Both the UV absorption at 254 nm and the visible
sensitive sensor for oxygen known so far exploits the efﬁcient absorption at 600 nm were studied. Sensing in the UV region
4272 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
allows for highly sensitive detectors due to its high absorptivity different thicknesses were prepared layer-by-layer, all showing a
in that region. The visible region has a signiﬁcantly lower linear sensitivity to NH3 in the 0-100 ppm range and a response
absorption coefﬁcient but enables monitoring high ozone levels. time of around 30 s. The sensor was regenerated by rinsing with
The UV based sensor can detect 0-1 mg/L of ozone and the water.
longwave sensor 25-126 mg/L. Most ammonia sensors reported so far make use of its effect
Nitrogen is a species not easily sensed by optical means except on appropriate pH probes. This is also true for one more sensor
by Raman spectroscopy. A respective FOCS to monitor the that makes use of a sol-gel matrix (76). Both NH3 vapor and
concentration ratio of nitrogen and oxygen in a cryogenic mixture trace NH3 dissolved in water can be detected. The indicator was
was reported by Tiwari et al. (69). Spontaneous Raman scattering immobilized in porous SiO2 which then was deposited on the
was used to monitor of the purity of liquid oxygen in the oxidizer surface of a bent optical ﬁber core. In combination with a silicone
feed line during ground testing of rocket engines. The Raman protection coating, ammonia can be sensed in watery samples.
peak intensity ratios for mixtures of liquid nitrogen and liquid The limit of detection is 13 ppb in gas samples and 5 ppb in water
oxygen were analyzed for their applicability to impurity sensing samples. This is comparable to previously reported probes
using different excitation light sources, and a miniaturized sensing exploiting the same effect. The scheme was extended to sensing
system was developed that responds within a few seconds. A ammonia via ﬁber-optic microsensors at 2-100 µg/L levels that
practical sensor for online sensing of atomic nitrogen in direct are known to be highly toxic to ﬁsh and other organisms (77). A
current glow discharges was reported by Popovic et al. (70). ﬂuorescent pH indicator placed in a cellulose ester matrix at the
Sensing is accomplished by monitoring the intensities of the tip of an optical microﬁber is deprotonated by ammonia, thereby
atomic nitrogen spectral line at 822 nm and the bandhead at 337 undergoing a large change in ﬂuorescence intensity. The resulting
nm, relative to the oxygen line at 845 nm. Measurements were ammonium ion is stabilized by a cation trap. The detection limit
performed using two methods. The ﬁrst approach uses a ﬁber- is reported to be 0.5 µg/L. The microsensors were used to
optic spectrometer, calibrated with a standard source, to record establish ammonia microproﬁles of high spatial resolution. Inter-
the desired spectral lines. The second approach uses narrow ference by trimethylamine was minimized using an 18-crown-6
bandwidth optical ﬁlters to detect the emission intensity. Optical ether as a cation trap and by the permeability properties of the
data are collected for a range of experimental conditions in a polymer matrix. Cross-sensitivities toward protons and alkali ions
ﬂowing glow discharge of N2-O2 mixtures. were prevented by coating the sensor with a layer of Teﬂon.
Sensors for carbon dioxide and, to a lesser extent, ammonia Ammonia also was sensed with the help of silica nanocom-
remain another active area of research. Carbon dioxide emissions posites doped with silver nanoparticles and deposited on an optical
from a diesel engine can be monitored with the help of a mid- ﬁber waveguide (78). The SiO2 nanocomposite was prepared by
infrared optical ﬁber sensor (71). As conventional automotive the sol-gel technique in the presence of (3-mercaptopropyl)tri-
pollution sensors fail to meet monitoring requirements as speciﬁed methoxysilane) and doped with 25 µm silver nanoparticles. The
by the European Community, for various reasons, a low-cost material was deposited onto the surface of a bent silica ﬁber by
sensor employing compact mid-IR components is presented that the dip-coating technique. Exposure to gas containing ammonia
was used to measure CO2 in the exhaust of a commercial diesel enhances the attenuation of light power guided through the ﬁber
engine. A novel kind of optical sensor for carbon dioxide makes probe. Sub-ppm levels of NH3 can be continuously monitored by
use of silicone encapsulated ionic liquids (72). The silicone matrix this technique.
acts as a permeation-selective material for CO2, while the ionic Oberg et al. (79) have designed a simple optical sensor for
liquid contains a pH probe (in its base form) that undergoes a vapors of organic amines based on silica microspheres dyed with
distinct color change after its (reversible) reaction with CO2. pH indicators such as bromocresol green (a probe reported to
Dissolved CO2 usually is sensed via the effect it exerts on the pH be useful for sensing ammonia by others several times before).
of a buffer immobilized in a matrix. A sulfonated hydroxypyrene It can detect organic amine vapors down to 1-2 ppb levels. The
named HPTS has been widely used as a pH probe in such CO2 response to aliphatic amines is linear up to 2 ppm. The micro-
sensors in the past two decades, and one more type of organically sphere sensor is said to be more sensitive than other optical amine
modiﬁed sol-gel was used to develop one more modiﬁcation of sensor described in the literature but heavily interfered by
this kind of sensor (73). Because the gel is partially ﬂuorinated, ammonia. The group of Mohr reports (80) that functional liquid
the response is faster. Nanoporous matrixes also may be used to crystal ﬁlms can selectively recognize amine vapors and thereby
host pH-sensitive phenolic dyes and an alkaline phase transfer undergo a change in their color. Molecular recognition is ac-
agent, reagents typically employed in sensors for CO2 (74). complished by cholesteric liquid crystals combined with molecules
Ammonia in the gas phase may be sensed either via its NIR carrying a triﬂuoroacetyl group. Note: sensors for amines in ﬂuid
absorption or (in being a weak base) via the weak pH changes it phases are treated in section D on sensors for organic molecules.
can induce in immobilized pH indicators. The latter approach is One more ﬁber-optic probe was reported for the selective
more sensitive. It also is the prefered one in that it is applicable determination of NO2 in air samples (81). Signal generation is
to aqueous solutions (unless extractive phases are employed in based on the spectroscopic changes of a reagent contained in the
IR sensing schemes). A new sensing scheme for ammonia is based nanopores of a sol-gel element that undergoes an irreversible
on monitoring the optical changes of the Q-band of tetrakis(4- chromogenic reaction. Two ﬁber-optic designs are described.
sulfophenyl)-porphine (TSPP) at 700 nm, as induced by ammonia Another FOCS for NO2 makes use of poly(3-octylthiophene) as a
in the electrostatic interaction between TSPP and poly(diallyldim- sensing material (82). This material undergoes large spectral
ethylammonium) chloride (75). Three thin ﬁlm samples with changes at 543 nm if exposed to NO2. The sensor is not very stable
Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4273
in that the response decreases after each exposure to NO2 but is (90) that uses birefringent porous glass oriented between two
rapid, highly selective, and sensitive. Vehicle exhaust emissions crossed polarizers and serving as the basis for this broad-spectrum
such as NO2, NO, SO2 were detected using an ultraviolet optical sensor. VOCs such as acetonitrile vapors can be detected at
ﬁber based sensor (83). It is used to simultaneously measure their concentrations of >50 ppm. The optical effects resulting from
concentrations, with lower limits of detection of 2 ppm for NO2, 2 exposure to various VOCs are reversible and may result from
ppm for SO2, and 20 ppm for NO. Response times are <4 s. Optical adsorption of VOCs with attendant reduction of anisotropy. The
sensitivity to NO2, carbon monoxide, and hydrogen (all in dry sensor can make use of ambient light as a light source and the
air) was observed for a layer of a metal oxide multilayer of the eye as a detector to register the resulting color changes and thus
type InxOyNz covered with gold nanoclusters (84). The incorpora- is capable of real time monitoring of VOCs. Also see ref 91.
tion of the gold nanoclusters leads to a broad absorption peak in A ﬁber-optic nanosensor for volatile alcoholic compounds has
the visible (purple) due to the excitation of localized surface been described by Elosua et al. (92). It is based on a new
plasmons. The maximum of the peak is shifted on exposure to vapochromic red powder consisting of a silver metal organic
oxidizing or reducing gases. compound. Its color and refractive index change when exposed
An irreversible active core ﬁber-optic probe was developed for to vapors of VOCs. The process is fully reversible. The sensor
determination of trace H2S at high temperature using a cadmium consists of a nanometer-scale Fizeau interferometer doped with
oxide doped porous silica ﬁber as a transducer (85). If exposed the vapochromic material and placed at the cleaved end of a
to a gas sample at high temperatures, trace H2S in the sample multimode ﬁber-optic pigtail. The response of the material toward
diffuses into the porous ﬁber and reacts with cadmium oxide to different alcohols was measured at 850 nm, and changes up to 3
form cadmium sulﬁde (CdS), and this is detected at around 370 dB in the reﬂected optical power were registered. The authors
nm with a ﬁber-optic spectrometer. The CdS formed also emits have used the same material in a Fabry-Perot interferometer with
strong ﬂuorescence, with a peak emission at around 500 nm, but a nanosized cavity along with a optical ﬁber pigtail system (93).
this signal is quenched at 450 °C, so that ﬂuorescence can only The performance of a carbon nanotube (CNT) based thin ﬁlms
be used for probing trace H2S at low temperature. Fuel gases such ﬁber-optic for VOCs was described (94). Single-walled CNT
as H2, CH4, and CO were found not to interfere. The toxic multilayer coatings were used that cause a response toward
industrial chemicals hydrogen cyanide, hydrogen sulﬁde, and toluene and xylene vapors. The Langmuir-Blodgett technique was
chlorine can be sensed with waveguides coated with doped applied to deposit the CNTs directly onto the optical and acoustic
polymer materials in the form of arrays (86). The polymers are sensors substrates. The results demonstrate ppm to ppb sensitiv-
patterned on glass substrates and undergo color changes that can ity, fast response, and high repeatability. In a related paper, the
be interrogated at different wavelengths. Miniature waveguide incorporation of CNTs into hollow core ﬁber optics is reported
channels result in enhanced sensitivity owing to the increased (95).
path length. The sensors have response times (t90) of less than King et al. (96) report on a microsensor for VOCs where a
20 s. Certain cross interferences observed can be eliminated by photonic crystal of porous silicone is attached to the distal end of
applying a signal processing algorithm that also reduces false an optical ﬁber. Activated carbon is used as a preﬁlter, and any
alarms. Elemental iodine, unlike chlorine, exerts a quenching breakthrough of the VOC through the carbon results a large
effect on a derivative of aminobenzanthrone, and this effect was change in the reﬂectivity of the porous silica.
used in a respective ﬁber-optic sensor (87). It can detect iodine Humidity. Optical humidity sensors are a kind of evergreen
with a rather high concentration detection limit of 6 µM. given their highly different applications. Numerous materials
An optical biosniffer for methyl mercaptan, one of the smelling including Naﬁon ﬁlms doped with crystal violet responding to
principles in halitosis, was reported (88). Monoamine oxidase humidity by a change in their reﬂectance (97). Relative humidity
(MAO) was immobilized at the tip of a ﬁber-optic oxygen sensor (RH) between 0 and 0.25% can be determined with the respective
with a luminescent oxygen probe (excitation at 470 nm, emission sensor, with a detection limit as low as 0.018% RH (equal to ∼4
peaking at 600 nm). The sensing scheme is based on the detection ppm). The response is fully reversible (with some hysteresis) in
of the oxygen consumed as a result of enzymatic oxidation of dry nitrogen. Reversal times depend on exposure time and % RH.
methyl mercaptan (MM) by MAO. The output of the sniffer was The sensor can detect moisture in process gases such as nitrogen
ampliﬁed by substrate regeneration via reduction with ascorbic and HCl. In another kind of RH sensor, Ru(II) complexes were
acid. MM levels between 9 and 11 000 ppb were detectable. The employed since their luminescence decay time depends on RH.
pathological threshold is 200 ppb, the limits of human perception The analytical information is obtained via phase-sensitive deter-
are on the order of 10 ppb. mination of the decay time (98). The ruthenium probe was
A rather complex method for optical sensing of sulfur dioxide immobilized on a Teﬂon support. The operational range is from
in wines (89) employs a dinuclear palladium ligand complex 4 to 100% RH at 20 °C. Its response time is shorter than 2 min
immobilized in a PVC membrane plasticized with o-nitrophenyl- (recovery time <1.2 min). Signal stability was veriﬁed for >2.5
octylether (o-NPOE). Both free and total SO2 can be determined. years of discontinuous measurement. The sensor was applied to
Linear responses up to 50 and 150 mg/L were obtained for free measure RH in food and in a weather station.
and total SO2, with detection limits of 0.37 and 0.70 mg L-1, Certain porphyrins deposited in Naﬁon ﬁlms undergo water-
respectively. induced tautomerism. This forms the basis of a novel type of ﬁber-
Vapors. Vapors, often referred to as volatile organic com- optic RH sensor (99). It exhibits long-term stability and a linear
pounds (VOCs), often are sensed by optical means in order to response over the humidity range from 0 to 4000 ppm. Another
reduce the risk of explosion. An FOCS for VOCs was reported ﬁber-optic RH sensor reported (100) is based on large-core
4274 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
quartz/polymer optical ﬁber pairs. Examples of vapors that cence lifetimes, photostabilities, and acidity constants were
interfere (or may be sensed with this device) include those of determined in organic solvents and in PVC. The Schiff bases can
acetone and ethanol. In other words: it is not speciﬁc for RH. be photoexcited at 556 and 570 nm, respectively, and respond to
The sensitivity of a tapered optical ﬁber relative humidity (RH) pH in the range from 8.0-12.0 and 7.0-12.0, respectively.
sensor was optimized by means of tuning the thickness of A novel wide pH range sensing system was described that is
nanostructured sensitive coatings (101). A single mode tapered based on a sol-gel encapsulated amino-functionalized corrole
ﬁber was coated with a speciﬁc nanostructured polymer whose (109). An amino modiﬁed ﬂuorescent aminophenylcorrole im-
thickness was controlled so to optimize sensitivity to RH. The mobilized in a sol-gel SiO2 matrix undergoes large changes in
sensor displays a 27 times better sensitivity to RH than a ﬂuorescence intensity owing to multiple steps of protonation and
comparable overlay, this enabling monitoring human breathing. deprotonation, thereby allowing larger pH ranges (1-11) to be
Huang et al. report (102) that a thermoplastic polyimide when covered than via respective tetraphenylporphyrins. Dong et al.
deposited on a ﬁber-optic Bragg grating can act as a material (110) have obtained wide-range pH optical sensor by immobilizing
sensitive to RH in that it undergoes reversible expansion and three indicators in a sol-gel matrix that was deposited in a ﬁber
shrinkage. optic waveguide and optically interrogated by evanescent wave
absorptiometry. The sensors have a dynamic range from pH 4.5
SENSORS FOR pH AND IONS to 13.0.
This section covers sensors for all kinds of inorganic ions
A scheme for measurement of very high pH values as they
including the proton (i.e., pH), cations, and anions. Optical sensing
occur in chemical processing was presented by Gotou et al. (111).
of pH remains to be of greatest interest even though all optical
It is used to establish a health monitoring method for chemically
sensors suffer from cross sensitivity to ionic strength. The number
exposed ﬁber reinforced plastic structures in that it can detect
of articles on pH sensors is decreasing, though, which does not
the penetration of corrosive solutions into plastics. A high-pH
come as a surprise in view of the state of the art and the fact that
indicator is used along with an optical ﬁber connected to a
certain pH FOS are commercially available. On the other side,
spectrophotometer.
sensing of pH values below 1 and above 12 still represents a
Seki et al. describe a pH sensor based on a heterocore
substantial challenge in terms of material sciences. Optical pH
structured ﬁber optic (112). It consists of multimode ﬁbers and
sensors respond over a limited range of pH only (in most cases).
a short piece of single mode ﬁber which was inserted into the
It also needs to be stated that many pH sensors presented in the
multimode ﬁbers. The pH indicator phenol red was immobilized
past few years are modiﬁcations only of existing sensing schemes
in a sol-gel matrix at the surface of the heterocore portion. pH
and materials, and that authors often do not properly cite previous
changes were detected by measuring the loss spectra at 575 and
work on the subject. In the worst case, sensors are presented
that are inferior to others described before. Such sensors are not 545 nm, respectively. It is noted at this point by the author of this
treated here. review that sol-gels and other condensates of that kind are known
The group of Scheper has introduced a scheme for referenced not to provide a temporally stable matrix but rather to change
sensing of pH that is based on spectral analysis of ﬂuorescence their microstructure over periods of typically a few months. This
(103). pH is calculated by chemometric means from selected data is rarely addressed by authors of respective work.
points of the ﬂuorescence spectra of aminoﬂuorescein at high Indicator loaded microbeads that are permeation selective for
concentration so that a strong inner ﬁlter is operative. Martin et either protons or dissolved oxygen were used in a respective
al. describe a new organic pH indicator dye (mercurochrome) dually sensing membrane and a single ﬁber-optic sensor (113).
that was immobilized in a sol-gel matrix placed at the end of a The pH probe (a carboxyﬂuorescein) and the oxygen probe (a
ﬁber optic and enables measurement of pH in the pH 4-8 range ruthenium complex) were incorporated into two kinds of micro-
(104). The sensor is constructed from low cost ﬁber optic and particles, viz. an amino-modiﬁed poly(hydroxyethyl methacrylate)
optoelectronic components including a blue light emitting diode and an organically modiﬁed sol-gel, respectively. Both kinds of
and a photodiode. The sensing scheme relies on the measurement beads were then dispersed into a hydrogel matrix and placed at
of the ﬂuorescence intensity of the pH indicator that is related to the distal end of an optical ﬁber waveguide for optical interroga-
the intensity of the blue excitation light reﬂected by the sensing tion. A phase-modulated blue-green LED served as the light source
phase. The ratio between the two signals is proportional to pH for exciting luminescence whose average decay times or phase
but independent of excitation light intensity. The applicability of shifts served as the analytical information. Data are evaluated by
this sensor was tested for its performance in pH analysis in tap a modiﬁed dual luminophore referencing method which relates
and bottled mineral water (105). The same group also has the phase shift (as measured at two different frequencies) to pH
developed sol-gel matrixes by controlled hydrolysis of a titanium and to O partial pressure.
tetraalkoxide (106). The resulting sol-gel (TiO2) is said to be In related work but using different materials (for optoelectronic
more resistant and to have a longer lifetime than SiO2 ﬁlms. The reasons), microsensors were described for simultaneous sensing
matrix was doped with various pH indicator dyes, each being of pH and oxygen (114). This is important in clinical chemistry
sensitive to different pH ranges. and if minute sample volumes are only available. The microsensors
Congo red and neutral red on a cellulose acetate support have with a tip diameter of ∼140 µm make use of luminescent
been reinvented as a material for sensing the 4.2-6.3 and 4.1-9.0 microbeads that respond to the respective parameters by a change
pH range, respectively (107). Two aromatic Schiff bases were in the decay time, intensity, or both. A complex algorithm enables
studied for their suitability in terms of sensing alkaline pH values precise calculation of pH even if the spectra of the indicator dyes
(108). Absorption and emission spectra, quantum yields, ﬂuores- (for pH and oxygen) overlap.
Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4275
An evanescent wave direct spectroscopic sensor for chromate a covalent bond between their boronic acid moiety and the diol
anion reported by Tao & Sarma (115) uses a ﬂexible fused silica moiety of saccharides. This causes their ﬂuorescence to increase.
light guiding capillary as a transducer. The capillary has a cladding The probe can be photoexcited at around 460 nm, emits with a
layer and a protective polymer coating on its outside surface. The peak at 600 nm, and can be used at near neutral pHs. For a review,
cladding layer ensures the ability of the capillary to guide light, on probes for saccharides and glycosylated biomolecules see ref
while the protective coating increases its mechanical strength. Like 125.
similar sensors described before, the sensor is based on the Tri-n-propylamine and the drugs benzhexol and procyclidine
intrinsic evanescent wave absorption by chromate ions in a water were determined via electrochemiluminescence (ECL) in a sensor
sample inside the capillary. A 30 m long capillary has the capability constructed by the screen-print technique (126). Ruthenium(II)-
of detecting as little as 31 ppm of chromate. tris(bipyridine) on Naﬁon was immobilized on a carbon electrode,
A ﬂuorescence-based calcium nanosensor was described (116) and its ECL resulting from the interaction with the analytes was
that exploits silica nanoparticles (prepared by inverse microemul- measured. Limits of detection are as small as 30 nM. Polymer
sion polymerization) doped with the calcium(II) probe calcein as membranes containing a chromogenic functional azo dye undergo
both a recognition and transduction element for optical determi- color changes on (reversible) interactions with amines in organic
nation of calcium in blood serum. Traces of vanadium(V) ion can solvents (127). The dye (covalently linked to the polymer matrix)
been determined by using an irreversible chromogenic reaction recognizes the analyte via covalent binding. Binding constants of
between vanadium ion and a hydroxamic acid immobilized in a the various amines depend on the kind of solvent and are highly
poly(vinyl chloride) (PVC) membrane (117). A quenchable different, this resulting in strongly varying limits of detection.
ﬂuorescent benzofurane derivative in the plasticized PVC matrix Dissolved amines also can be sensed by incorporating an amine-
served as the indicator dye in an FOCS for ferric ion (118). carrying chromoionophore into a sol-gel matrix (128). Both acid-
Response time, reversibility, limits of detection (poor), dynamic and base-catalyzed sol-gel processes were studied, but the former
range, and interferences were studied. The same group has were found not to respond at all. Stable ormosil layers were
synthesized a ﬂuorescent semicarbazone and demonstrated its obtained using various fractions of organically modiﬁed sol-gel
applicability as a selective probe in an FOCS for copper(II) (119). precursors, e.g., methyl triethoxysilane. Base-catalyzed sensor
PVC and ethyl cellulose acted as sensor matrixes, and both layers underwent large signal changes, with response times of
absorption and emission spectrometry can be applied. Aluminum around 1-2 min, and rather high detection limits (0.1 mM).
ion in aqueous media can be probed with a regenerable sensor Organics. A ﬂuorescent dosimeter for formaldehyde deter-
that uses a molecularly imprinted polymer (MIP) as the recogni- mination in water utilizes the Nash reagent incorporated into silica
tion receptor (120). The MIP was prepared with Al(III) ion acting gel beads (129). On reaction with formaldehyde, a ﬂuorescent
as the template, and 8-hydroxyquinoline sulfonate acted as a product is formed that can be detected instrumentally or visually.
ﬂuorogenic ligand. The MIP was synthesized from acrylamide, The dosimeter does not respond to primary alcohols, ketones,
2-hydroxyethyl methacrylate, and ethylene glycol dimethylacrylate and other common substances. The detection limit is reported to
as a cross-linker. At pH 5, Cu(II) and Zn(II) interfere to some be 30 µg/L. Dissolved organics in water samples can be sensed
extent. The dynamic range at pH 5 is linear up to 0.1 mM, the with a nanoporous zeolite thin ﬁlm-based ﬁber sensor (130). A
limit of detection is 4 µM. Mercury(II) ions were optically probed Fabry-Perot interferometric system was developed containing a
by Li et al. (121). They report on a luminescent nanosensor for thin layer of nanoporous zeolite synthesized on the cleaved end
Hg(II) where the ﬂuorescence of carnitine capped quantum dots face of a single mode ﬁber. The sensor is operated by monitoring
made from CdSe/ZnS is quenched by Hg(II) ions with an changes in the thickness of the thin ﬁlm caused by the adsorption
efﬁciency that resulted in a detection limit of 0.2 µM. In related of organic molecules by white light interferometry. Methanol,
work it is reported (122) that Pb(II) ions can be (irreversibly) 2-propanol, and toluene were detected with high sensitivity. A
probed by a similar method but using CdTe quantum dots capped FOCS for toluene in water was described by Consales et al. (131).
with thiols. The detection limit is virtually the same. It makes use of single-walled carbon nanotubes whose reﬂectivity
changes in the presence of toluene. System features include good
SENSORS FOR ORGANIC CHEMICALS stability of the steady-state signal, sensitivity, and rapid response.
This chapter covers sensors for organic species (mainly
saccharides), pollutants, agrochemicals, nerve agents, drugs and BIOSENSORS
pharmaceuticals, and miscellaneous other organics. Mid-IR laser This section covers biosensors based on the use of enzymes,
spectroscopy was performed by either using cryogenically cooled antibodies, nucleic acids, and whole cells. Biosensors make use
lead salt lasers or quantum cascade lasers operating at room of biological components in order to sense a species of interest
temperature and applied to reagentless (enzymeless) sensing of (which by itself need not be a “biospecies”). On the other side,
glucose (123). Aqueous solutions of glucose were analyzed by chemical sensors not using a biological component but placed in
ﬁber based attenuated total reﬂection spectroscopy and by a biological matrix are not biosensors by deﬁnition. It should be
transmission spectroscopy. Both methods have the potential to noted that some of the biosensors can be found in other chapters
be utilized in small ﬁber sensors that may be inserted into if this was deemed to be more appropriate.
subcutaneous tissue. Concentrations as low as 0.1 mg/mL were Enzymatic Biosensors. Sensing glucose remains to be an
detectable. evergreen. Unlike in the case of electrochemical schemes where
Mohr et al. (124) describe ﬂuoro-reactands for the enzymeless direct electron shuttle from the substrate to the electrode has
determination of saccharides. They are based on hemicyanine become possible as a result of enzyme wiring, no such approach
dyes containing a boronic acid receptor and are capable of forming is possible in optical sensors. Hence, these still rely on (a)
4276 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
transduction via metabolic coreactants (such as oxygen or NAD+); the ﬂuorescent pH indicator carboxynaphthoﬂuorescein. Hydroly-
(b) of coproducts (such as protons, hydrogen peroxide, or NADH); sis of organophosphates by OPH causes the pH to fall, and this
or (c) on afﬁnity binding (such as to concanavalin A). Compre- is reported by the pH probe. The dynamic range for determination
hensive reviews have appeared (132). The group of Klimant has of paraoxon is from 1 to 800 µmol/L.
designed a ﬁber-optic ﬂow through sensor for online monitoring Single molecules of galactosidase were monitored (140) using
of glucose in patients in intensive care units (133). A tubing as a 1 mm diameter ﬁber-optic bundle with individually sealed
used in microdialysis contains an integrated ﬁber-optic sensor. femtoliter microwell reactors. Unlike in so-called “ensemble”
Glucose is sensed via oxygen consumption which occurs as a responses in which many analyte molecules give rise to the
result of the oxidation of glucose catalyzed by immobilized GOx. measured signal, the buildup of ﬂuorescent products from single
The gas permeable tubing warrants constant air saturation in the enzyme molecules catalysis over the array of reaction vessels can
ﬂow cell. A reference oxygen sensor is used to detect changes in be observed, and a digital concentration readout can be obtained
oxygen supply caused by adverse effects such as bacterial growth, by application of statistical analysis. This approach should prove
temperature ﬂuctuations, or failure of the peristaltic pump. The useful for single molecule enzymology and ultrasensitive bioas-
sensor was evaluated in a 24 h test on a healthy volunteer. Endo says. A similar approach (141) was applied to 24 000 individual
et al. describe a needle-type enzyme sensor system for determin- reaction chambers to sense DNA and antibodies and again is
ing glucose levels in ﬁsh blood (134). A hollow needle was used expected to be applicable to assays that utilize an enzyme label
that also was comprised of an immobilized enzyme membrane to catalyze the generation of a ﬂuorescent signal.
and a optic ﬁber probe with a quenchable ruthenium-based oxygen A ﬁber-optic enzymatic biosensor was described for determi-
indicator. The calibration curve was linear for glucose in ﬁsh nation of 1,2-dichloroethane (DCA) (142). Haloalkane dehaloge-
plasma. One assay was completed within 3 min. A good reproduc- nase in whole cells of Xanthobacter autotrophicus immobilized in
ibility was observed 60 times without exchange of the enzyme calcium alginate at the tip of a ﬁber-optic coverts the haloalkanes
membrane. The group of Nann (135) found that the luminescence into acidic products (i.e., lowers the pH), and this is detected via
of silica coated quantum dots is dynamically quenched by the pH probe ﬂuorescein immobilized at the tip of the FOBS. DCA
hydrogen peroxide (H2O2), and this was exploited in a optical was quantiﬁed at levels as low as 11 mg/L, with a linear response
glucose assay using the enzyme GOx. This is one of the few at ∼65 mg/L. Like most cell-based catalytic biosensors, the
reversible optical methods that are based on the transduction of response time is slow (8-10 min). Rajan et al. report on the
oxidase based reactions via H2O2. The quantum dots have a rather fabrication and characterization of a surface plasmon resonance
high speciﬁc surface area which enables a relatively large amount based enzymatic FOBS for detection of the bittering component
of GOx to be immobilized. On the basis of the previous work on naringin (143). The sensing area was coated over ∼1 cm length
sensing H2O2 via the amount of oxygen formed by catalytic with silver and then with the enzyme naringinase. The SPR
decomposition of H2O2, Mills et al. (136) have designed a robust wavelength maximum increases with the concentration of naringin.
and reversible sensor where ruthenium(IV)dioxide is used as a Immunosensors. Antinuclear antibodies can be quantiﬁed
catalyst. The amount of oxygen liberated is determined via the with an optical ﬁber biosensor modiﬁed with colloidal gold (144).
quenching effect it exerts on the luminescence of a ruthenium(II) The unclad portion of an optical ﬁber was covered with self-
ligand complex. assembled gold colloids whose surface was functionalized with
Luminescent yeast cells were entrapped in hydrogels in order antigens. Antinuclear antibodies in serum can be determined
to optically detect estrogenic endocrine disrupting chemicals quantitatively, and the results agree well with the accepted ELISA
(137). Genetically modiﬁed Saccharomyces cerevisiae cells contain- method. This sensing platform is label-free, enables real-time
ing the estrogen receptor expression of the luc reporter gene were detection, and does not require a secondary antibody. Its sensitiv-
immobilized in hydrogel matrixes. The chemicals induce a ity is higher by at least 1 order of magnitude than that of the
chemiluminescence to be emitted by the cells. Concentrations ELISA method.
down to 80 ng/L of the chemicals were detectable. The probe Antibodies against the F1 antigen of Yersinia pestis (the cause
was stored for 1 month at -80 °C but full activity was attained for plague and a potential terroristic agent) can be detected by a
again at room temperature. The data obtained with this sensor sandwich immunoassay on the surface of an optical ﬁber (145).
roughly correlated with LC-MS/MS analytical results. Autoantibodies to ovarian and breast cancer-associated antigens
Fiber optic absorbance spectroscopy was compared with are detectable with high sensitivity by using a chemiluminescence
surface plasmon optical detection methods for lactamases bound based optical ﬁber immunosensor (146). The protein GIPC-1 was
to interfacial structures via biotin-avidin coupling (138). Quantita- conjugated to the tip of an optical ﬁber. A human monoclonal IgM
tive comparisons were made between the ﬁve matrixes and isolated from breast cancer patients targets the GIPC-1 protein
between the binding strategies. All matrixes were suited, in and thus can be detected in concentrations down to 30 pg/mL,
principle, for the binding of the protein. Results obtained by SPR which is 50 times lower than the chemiluminescent ELISA and
and optical waveguide measurements correlate excellently. Real ∼500 times lower than the colorimetric ELISA. Sera from 11
time activity assays of -lactamase were performed by a detection ovarian cancer patients, 22 breast cancer patients, and asymp-
scheme that combines an afﬁnity test and a catalytic sensor. tomatic controls were tested for the presence of IgM anti-GIPC-1
Organophosphate pesticides and chemical warfare agents can autoantibodies by the two methods.
be sensed with a FOBS that exploits the enzyme organophosphate A waveguide based immunosensor for the food toxins aﬂatoxin
hydrolase (OPH) as the biorecognition element (139). Conjugated B1 and ochratoxin A uses either the competitive or the direct
to biotin, it was attached to a polystyrene waveguide along with immunoassay format (147). The antibody conjugate was im-
Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4277
mobilized on a sensor chip and exposed to the analytes in a ﬂow Surface plasmon resonance (SPR) spectroscopy was used for
injection analyzer. The detection range of the competitive detection detecting target sequences in genomic DNA differing in terms of
method was between 0.5 and 10 ng/mL. The indirect method was copies in the relative genome (154). Following fragmentation with
used to sense toxins in barley and wheat ﬂour samples. Results restriction enzymes and denaturation, the interaction of the two
are in good agreement with those obtained by ELISA. strands is found to be speciﬁc both with oligonucleotides and
DNA Biosensors. A ﬁber-optic DNA microarray for simulta- with genomic nonampliﬁed DNA. The usual ampliﬁcation step
neous determination of multiple harmful algal bloom species was is found not to be necessary. In another type of DNA biosensor,
reported by Ahn et al. (148). Algal blooms are a serious threat to localized SPR spectroscopy was coupled to interferometry
coastal resources, causing a variety of impacts on public health, (155). A gold layer was deposited on porous anodic alumina
regional economies, and ecosystems. The sandwich hybridization and interrogated by both interferometry and localized SPR. The
assay combines ﬁber-optic array technology with appropriate intensity of light reﬂected by the chip resulted in an optical
oligonucleotide probes immobilized on microspheres. Hybridiza- pattern that was highly sensitive to changes in the effective
tion was visualized using ﬂuorescently labeled secondary probes. thickness of the layer on the surface, speciﬁcally oligonucle-
A detection limit as low as 5 cells is achieved, the assay time being otides and hybridized oligonucleotides.
45 min without a separate ampliﬁcation step. An elegant diagnostic In a novel kind of DNA biosensor, the electrochemilumines-
device for the detection of the hepatitis B virus was presented cence (ECL) of the Ru(II) bipyridyl complex (a weak intercalator)
(149). An isothermal ampliﬁcation method is employed so to is used to generate optical analytical information (156). If ds-DNA
detect the viral DNA in a 25 µL reaction volume following several binds to intercalators such as doxorubicin or daunorubicin, an
automated handling steps. easily detectable ECL is generated in the presence of the
Bacterial transcriptional regulators are known to be dose- ruthenium complex at +1.19 V (vs Ag/AgCl), while ss-DNA
dependently released from their operators upon binding of speciﬁc (which is not intercalated) does not give this effect. Given the
classes of antibiotics. In another approach, Link et al. (150) use sensitivity of ECL-based methods, this approach may be quite
a generic dipstick-based technology for rapid determination of powerful. Several pathogens (including hepatitis virus) were
tetracycline, streptogramin, and macrolide antibiotics in food detected by this technique.
samples. The dipstick assay consists of a membrane support strip Bacterial Biosensors. Bioavailable mercury and arsenic in
coated with streptavidin and immobilized biotinylated operator soil and sediments were determined with ﬁber-optic bacterial
DNA, which acts as capture DNA to bind hexa-histidine (His6)- biosensors (157). Alginate-immobilized recombinant luminescent
tagged bacterial biosensors. Antibiotics present in speciﬁc samples bacteria were immobilized on optical ﬁbers and enabled lumines-
triggered the dose-dependent release of the capture DNA-biosensor cent quantiﬁcation of 2.6 µg/L of Hg(II), 141 µg/L of As(V), and
interaction. Dipping the stick into two reagent solutions results 18 µg/L of As(III). The pesticide methyl parathion was shown to
in a correlated conversion of a chromogenic substrate by a His6- be detectable using Flavobacterium sp. adsorbed on glass ﬁber
targeted enzyme complex. It has detection limits as low as 1/40 ﬁlters as a disposable biocomponent (158). The ﬂavobacterium
of the licensed threshold. In a comparable approach, antibiotics contains a hydrolase that hydrolyzes methyl parathion into
in seafood were screened for a ﬁber-optic ﬂuorometric assay based optically detectable p-nitrophenol. Only 75 µL of sample are
on competitive binding of the analyte and an intercalating dye to needed to detect 0.3 nmol/L concentrations of methyl parathion.
double stranded DNA (151). The antibiotics affect the binding of Microbially available dissolved organic carbon was quantiﬁed
the intercalator to the double stranded nucleic acid, thereby with a microsensor (159) containing microorganisms in a poly-
changing the ﬂuorescence intensity. The concentration of the urethane hydrogel. A strain was used that displays low substrate
analyte is indirectly determined by the decrease in ﬂuorescence selectivity and responded to mono- and disaccharides, to fatty
intensity. A DNA of 48.5 kb was found to be a suitable sensing acids, and to amino acids. The 90% response time was 1-5 min.
nucleic acid. Sulfathiazole and chloramphenicol in shrimps were Another optical ﬁber biosensor for biochemical oxygen demand
sensed by this method with detection limits from 0.5 to 1 ng/mL. (BOD) is based on microorganisms coimmobilized in an ormosil
An optical biosensor for real-time detection of DNA interactions matrix (160). The consumption of dissolved oxygen is measured
was demonstrated with a long-period ﬁber-grating biosensor (152). with a ﬂuorescent optical ﬁber sensor. The BOD values obtained
The probe DNA was immobilized on the silanized surface of the correlate well with those determined by the conventional BOD5
grating and then hybridized with the complementary (target) method for seawater samples.
DNA. The sensor is reusable because the target DNA can be Genetically engineered bioluminescent bacteria were applied
stripped off the grating surface after the assay. Wang et al. report to the detection of toxicants in surface waters (161). The
on a DNA detection protocol utilizing the opacity of self-assembled multichannel system developed is based on mini-bioreactors
nanometallic particles (NPs) and the optical response of a CMOS containing four kinds of recombinant bioluminescent bacteria and
image sensor (153). The authors exploit the fact that DNA is connected to a luminometer via a ﬁber-optic cable. The system
fragments attached to NPs precipitate them only at locations can be continuously operated due to the separation of the bacteria
where cDNA strands exist. Hence, the opacity of the chip surface culture reactor from the test reactor without system shutdown
changes due to the accumulation of NPs and this can be used to by abrupt inﬂows of severe pollutants. Bioluminescent signatures
detect targeted DNA fragments. The approach is very sensitive, were delivered from four channels by switching one at once. The
detecting even single-base mismatched DNA targets in concentra- system is now being implemented to a drinking water reservoir
tions down to 10 pM. and river for remote sensing as an early warning system.
4278 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
APPLICATIONS Reﬂectance spectra in the 1250-2500 nm region for the skin of
This section comprises sensors for environmental, industrial, volunteers reveal large regional differences of water content. There
biotechnological, food, pharmaceutical, medical, and related ap- was a difference in the ratio of the two water bands centered near
plications of FOCS and FOBS. Thus, a newly developed miniatur- 1450 and 1900 nm between the contact and noncontact measure-
ized diamond ATR probe has been described that displays high ments. Most regional differences of water content were calculated
chemical and pressure resistance. In combination with robust and from the peak height of the 1900 nm water band normalized to
ﬂexible ﬁber-optics, it enables inline chemical reaction monitoring the peak height of the 2175 nm amide band.
(162). One large area of application of FOCS is in monitoring the
Huber et al. (163) reports on the measurement of the ingress mechanical and chemical integrity of concrete structures. Aside
of oxygen into PET bottles using oxygen sensor technology. A from sensing mechanical integrity (not treated here), changes in
noninvasive ﬁber-optic oxygen meter detects oxygen permeability chemical composition are of high interest given the health risk
of PET bottles for soft drinks by measuring trace levels of oxygen and costs associated with disintegrated structures. Optical ﬁber
inside the bottle. Permeation rates are obtained without piercing sensors have been used to monitoring the ingress of moisture in
the package or bottle which is ideally suited for assurance, structural concrete (170). The humidity sensors were fabricated
production, and quality control applications. Sensing is based on using ﬁber Bragg gratings coated with moisture sensitive poly-
quenching of the ﬂuorescence by oxygen of a sensor spot placed mers and are employed in detecting the movement of moisture
on the inner wall of the transparent bottle. A ﬁber-optic cable is through standardized cubes made from samples of different types
positioned outside and measures changes in luminescence life- of structural concrete. Data obtained can give information on the
time. Gaseous or dissolved oxygen can be detected in the ppm to properties of different types of concrete but also on the migration
ppb range. of dissolved salts, such as sodium chloride which is important in
view of their deleterious effects on reinforcement bars within
Near-infrared reﬂectance spectroscopy was applied to quanti-
concrete. The optical ﬁber sensors reacted to the ingress of water
tate Ca, K, P, Fe, Mn, Na, Zn, protein, and moisture in alfalfa (164).
by detecting the moisture migrating through the sample, indicated
The method allows immediate analysis of alfalfa without prior
by a shift in the Bragg wavelength of the sensor. A similar
sample treatment. A partial least-squares regression method was
approach was introduced by Yeo et al. (171).
employed. The prediction capacity of the model and the robust-
Chloride ion in concrete can be sensed with a long-period ﬁber
ness of the method were checked in the external validation in
grating (LPFG) (172). The LPFG device is sensitive to the
alfalfa samples of unknown compounds.
refractive index of the medium around the cladding surface of
The biocompatibility of Te-As-Se (TAS) glass ﬁbers for use
the sensing grating, thus offering the possibility of detecting a
in infrared sensors was studied by Wilhelm et al. (165). The ﬁbers
change in chemical concentration. The authors measured chloride
are used for IR direct spectroscopy of cultivated mammalian cells.
ions in a typical concrete sample immersed in salt water solutions
TAS ﬁbers undergo oxidation on air to form a water-soluble
in concentrations ranging from 0 to 25%. The sensor exhibited a
nanometer-thin layer that is soluble in water. The glass underneath
linear decrease in the transmission loss and resonance wavelength
this layer is, however, stable in water over several days. Hence,
shift when the chloride was increased. The limit of detection for
oxidized ﬁbers that can release arsenate ions are toxic, while
chloride ions is ∼0.04%. Sensitivity was further enhanced by
freshly prepared or washed ﬁbers are not. Glasses displaying less
coating a monolayer of colloidal gold nanoparticles as the active
strong interfering vibrations in the 2-5 µm spectral region were
material on the grating surface of the LPFG which increase the
prepared from TeO2-BaO-Bi2O3 mixtures (166). IR and Raman
sensitivity by a factor of 2.
spectroscopic studies were carried out, and the temperature
pH sensors for concrete are needed to detect any (highly
dependence of the viscosities of the glasses is reported.
adverse) changes in the rather high pH (>11) of concrete. An
A ﬁber-optic sensor for measurements of interstitial pH was
ﬁber-optic pH sensor was developed that can be incorporated into
further improved (167). The interstitial sample ﬂuid drawn
concrete and thus is capable of early detection of the danger of
subcutaneously from adipose tissue ﬂows through a microﬂuidic
corrosion in steel-reinforced concrete structures (173).
circuit formed by a microdialysis catheter in series with a glass
capillary. The pH indicator phenol red is covalently immobilized SENSING SCHEMES AND SPECTROSCOPIES
on the inner wall of the capillary. Optical ﬁbers are used to connect This section reports on improved or novel sensing schemes
the interrogating unit to the sensing capillary. A resolution of 0.03 based on the use of ﬁber optics and related waveguides. Aside
pH units and an accuracy of 0.07 pH units were obtained. from their use as plain waveguides, ﬁbers have been used for
Preliminary in vivo tests were carried out in pigs with altered evanescent wave excitation of ﬂuorescence or Raman scatter, for
respiratory function. imaging and sensor array purposes, in microsensors and nanosen-
Data on pH, carbon dioxide, and oxygen as obtained with ﬁber- sors, and for distributed sensing, to mention only the more
optic sensors on intramuscular and venous blood during rhythmic important ones. The current success of (ﬁber optic) surface
handgrip exercise were compared (168). A ﬁber-optic sensor was plasmon resonance (SPR) is obvious.
inserted into muscle for continuous measurement of intramuscular Fiber Optics. New in-line ﬁber-optic structures for environ-
data. Blood samples were taken from the forearm every minute mental sensing applications were described (174). Sensors based
during each exercise bout. The data for venous and arterial blood on the interaction of surface plasmons or evanescent waves with
were found to be highly different when exercising. the surrounding environment are usually obtained by tapering an
Regional differences in the water content of human skin can optical ﬁber. A ﬁber-optic structure is presented that maintains
be studied by diffuse reﬂectance near-infrared spectroscopy (169). the structural integrity of the optical ﬁber. Graded index optical
Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4279
ﬁber elements are used as lenses, and a coreless optical ﬁber acts Analog signal acquisition from computer optical disk drives
as the interaction area. These elements are fused by an optical was demonstrated to be useful in chemical sensing (182). Signals
ﬁber splicer. Two optical system designs were compared for ﬁber- were obtained from optical sensor ﬁlms deposited on conventional
optic chemical sensor applications (175). A single grating spec- CD and DVD optical disks. Almost any optical disk can be
trograph with ﬁber-optic input and photodiodes at three different employed for deposition and readout of sensor ﬁlms. The disk
wavelengths was compared to a system comprising 1-3 ﬁber- drives also perform the function of reading and writing digital
optic splitters and photodiode detectors with integrated interfer- content to optical media. Such a sensor platform is quite universal
ence ﬁlters. Three types of splitters were tested, and it is found and can be applied to sensing and combinatorial screening.
that that the systems have similar characteristics, also if used in Speciﬁcally, colorimetric calcium-sensitive ﬁlms were deposited
a colorimetric CO2 sensor. onto a DVD, exposed to different concentrations of Ca(II), and
Capillary Waveguides. Tao et al. (176) described the ap- quantiﬁed in the optical disk drive.
plication of a light guiding ﬂexible tubular waveguide in evanescent Ink jet printing technology was applied to fabricating micro-
wave absorption based sensing. A light guiding ﬂexible fused silica sized optical ﬁber imaging sensors (183). An array of photopo-
capillary (FSCap) was used that is similar to a conventional silica lymerizable sensing elements containing a pH sensitive indicator
ﬁber in that it can guide light in the wavelength region from UV was deposited on the surface of an optical ﬁber image guide. The
to near IR. The inner surface of the FSCap capillary was coated reproducibility of the microjet printing process was found to be
with a reagent doped polymer to design a FOCS. Techniques were excellent for micrometer-sized sensor spots. Hanko et al. (184)
developed for activating the inner surface of an FSCap, coating showed that nanophase-separated amphiphilic networks represent
the inner surface of the FSCap with a polymer, connecting the versatile matrixes for optical chemical and biochemical sensors.
coated FSCap to a light source and a photodetector, and delivering They consist of nanosized domains of hydrophilic and hydrophobic
a sample through the FSCap were developed. Sensors for Cu(II), polymers (comparable to a polyacrylamide-co-polyacrylonitrile
copolymer referred to as Hypan and previously introduced by
toluene in water samples and ammonia in a gas sample were
others). Because of the spatial separation, there are domains in
fabricated and tested. Similarly, the waveguiding properties of a
which the indicator reagents are immobilized and domains where
FCSap for chemical sensing applications were investigated by
diffusive transport of the analyte occurs. Various prototypes of
Keller et al. (177). Absorbance within the tubing was measured
sensors were prepared, e.g., for sensing gaseous chlorine (based
by optically coupling the FSCap to a spectrophotometer. The
on a chromogenic reaction), vapors of acids (based on immobilized
FSCap operated evanescently or as a liquid core waveguide
bromophenol blue), and peroxides (based on immobilized horse-
depending upon the refractive index of the sample solution within
radish peroxidase and a chromogenic substrate).
the capillary. Evanescent absorbance was linear with the concen-
Refractive Index. High-sensitivity optical chemosensors were
tration of a nonpolar dye but nonlinear with ionic dyes due to
implemented by exploiting ﬁber Bragg grating structures in
adsorption to the capillary wall. Absorbance measurements in 50,
D-shape, single-mode, and multimode ﬁbers and postsensitized
150, and 250 µm inner diameter FSCaps show that greater
by HF etching treatment (185). Hence, the intrinsically insensitive
sensitivity is achieved in thinner walled tubings because of more
Bragg grating became sensitive to refractive index (RI). The
internal reﬂections. A FSCap for pH is demonstrated.
resulting devices were used to measure the concentrations of
A liquid-ﬁlled hollow core microstructured polymer optical ﬁber
sugar solutions. A self-temperature-referenced sensor based on
(178) is said to be opening up many possibilities in FOCS. It is
nonuniform thinned ﬁber Bragg gratings was described (186).
demonstrated how the band gaps of such a hollow core polymer The sensor consists of a Bragg grating where the cladding layer
optical ﬁber scale with the refractive index of a liquid introduced is removed along half of the grating length. This perturbation leads
into the holes of the microstructure. The ﬁber is then ﬁlled with to a wavelength-splitting in two separate peaks: the peak at lower
an aqueous solution of (L)-fructose, and the resulting optical wavelengths corresponds to the thinned region and is dependent
rotation is measured. Hence, hollow core microstructured polymer on the outer RI and the local temperature, while the peak at longer
optical ﬁbers can be used for sensing chiral species. A distributed wavelength responds to thermal changes only. The sensor was
ﬁber-optic polarimetric sensor was reported by Caron et al. (179). characterized in terms of thermal and RI sensitivities. A photonic
The sensor is based on evanescent wave polarimetric interferom- band gap ﬁber for measurement of RI was described by Sun and
etry and is intended for use in gas chromatography. It allows real- Chan (187).
time monitoring of the displacement of a chemical substance along Spectroscopies. Fiber optic (bio)sensors were reported that
a capillary. are based on localized surface plasmon resonance (SPR) (188).
Microsystems and Microstructures. A sub-nanoliter spec- The sensor measures the light intensity of the internally reﬂected
troscopic gas sensor was described (180) and compared to light at a ﬁxed wavelength from an optical ﬁber where the
existing sensors designs. This novel gas sensor has the capability extinction cross-section of self-assembled gold nanoparticles on
of gas detection with a cell volume in the sub-nanoliter range. A the unclad portion of the optical ﬁber changes with the refractive
study of the capabilities of microstructure ﬁbers for evanescent index of a sample near the gold surface. Sensing of the Ni(II) ion
wave vapor sensing is presented in ref 181. Toluene vapor in and label-free detection of streptavidin and staphylococcal entero-
nitrogen gas was investigated. It causes a change in the refractive toxin B is demonstrated at picomolar levels. A related reﬂection
index changes of the xerogel ﬁber cladding at 670 nm. Moreover, based localized SPR ﬁber-optic probe was developed to determine
speciﬁc changes in absorbance due to C-H overtone absorptions refractive indexes and, thus, chemical concentrations at high
of toluene at 1600-1800 nm were exploited. pressure conditions (189). Sensing is based on the measurement
4280 Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
of the intensity of internal light reﬂection at a ﬁxed wavelength acted as the (co)organizer of several conferences related to ﬂuorescence
from an optical ﬁber. The light attenuation caused by the spectroscopy (MAF) and to chemical sensors and biosensors (Europtrode).
He acts on the board of several journals including Angewandte Chemie
absorption of self-assembled gold nanoparticles on the unclad and is the Editor-in-Chief of Microchimica Acta. His research interests
portion of the optical ﬁber changes with a different refractive index are in optical chemical sensing and biosensing, in ﬂuorescent probes and
of the environment near the gold surface. The probe demonstrated labels, in ﬂuorescence-based analytical formats including imaging, in
biosensors based on thin metal ﬁlms using capacitive or SPR interrogation,
a stable and repeatable response for sequential operations of and in the design of advanced (nano)materials (including ﬂuorescence
pressurization and depressurization at 0.1-20.4 MPa at 308 K. A upconverters) for use in (bio)chemical sensing.
new concept of an SPR ﬁber-optic sensor was presented (190). A
signiﬁcant variation of the spectral transmittance of the device is
LITERATURE CITED
produced as a function of the concentration of the analyte by
tuning the plasmon resonance to a wavelength for which the outer BOOKS AND REVIEWS
medium is absorptive. With this mechanism, selectivity can be (1) Wolfbeis, O. S. Anal. Chem. 2002, 74, 2663–2677.
(2) Wolfbeis, O. S. Anal. Chem. 2004, 76, 3269–3284.
achieved without the need of any functionalization of the surfaces
(3) Wolfbeis, O. S. Anal. Chem. 2006, 78, 3859–3873.
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for any kind of FOCS or FOBS. 400–422.
Cavity ringdown (CRD) absorption spectroscopy enables spec- (5) Eckhardt, H. S.; Klein, K.-F.; Spangenberg, B.; Sun, T.; Grattan, K. T. V.
Journal of Physics: Conference Series; 2007; Vol. 85, no pages given, on-
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limits of sensitivity were further pushed (191). This continuous-wave (6) Buchanan, B. Practical Spectroscopy. Handbook of Near-Infrared Analysis,
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C.; Nagl, S.; Schaeferling, M.; Stich, M. I.; Kocincova, A. S.; Wolfbeis,
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A luminescent ratiometric method in the frequency domain New York, 2008; pp 431-463, DOI: 10.1007/4243_013.
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