Selective Detection of Erythrocytes with QCMs-ABO Blood Group Typing

Blood transfusion, as well as organ transplantation, is only possible after prior blood group (BG) typing and crossmatching. The most important blood group system is that of Landsteiner's ABO classification based on antigen presence on the erythrocyte surfaces. A mass sensitive QCM (quartz crystal microbalance) sensor for BG typing has been developed by utilizing molecular imprinting technology. Polyvinylpyrrolidone (crosslinked with N,N-methylenebisacrylamide) is a favorable coating that was imprinted with erythrocytes of different blood groups. In total, 10 MHz quartz sheets with two resonators, one for MIP (molecularly imprinted polymer) and the other for NIP (non-imprinted polymer) were fabricated and later used for mass-sensitive measurements. The structure of erythrocyte imprints resembles a donut, as identified by AFM (atomic force microscope). All the erythrocytes of the ABO system were chosen as templates and the responses to these selective coatings were evaluated against all blood groups. Each blood group can be characterized by the pattern of responses in an unambiguous way. The results for blood group O are remarkable given that all types of erythrocytes give nearly the same result. This can be easily understood as blood group O does not possess neither antigen A nor antigen B. The responses can be roughly related to the number of respective antigens on the erythrocyte surface. The imprints generate hollows, which are used for reversible recognition of the erythrocytes. This procedure is based on molecular recognition (based on supramolecular strategies), which results from size, shape and enthalpic interactions between host and guest molecules.

Transitioning from Supramolecular Chemistry to Molecularly Imprinted Polymers in Chemical Sensing

This perspective article focuses on the overwhelming significance of molecular recognition in biological processes and its emulation in synthetic molecules and polymers for chemical sensing. The historical journey, from early investigations into enzyme catalysis and antibody-antigen interactions to Nobel Prize-winning breakthroughs in supramolecular chemistry, emphasizes the development of tailored molecular recognition materials. The discovery of supramolecular chemistry and molecular imprinting, as a versatile method for mimicking biological recognition, is discussed. The ability of supramolecular structures to develop selective host-guest interactions and the flexible design of molecularly imprinted polymers (MIPs) are highlighted, discussing their applications in chemical sensing. MIPs, mimicking the selectivity of natural receptors, offer advantages like rapid synthesis and cost-effectiveness. Finally, addressing major challenges in the field, this article summarizes the advancement of molecular recognition-based systems for chemical sensing and their transformative potential.

An Overview of High Frequency Acoustic Sensors-QCMs, SAWs and FBARs-Chemical and Biochemical Applications

Acoustic devices have found wide applications in chemical and biosensing fields owing to their high sensitivity, ruggedness, miniaturized design and integration ability with on-field electronic systems. One of the potential advantages of using these devices are their label-free detection mechanism since mass is the fundamental property of any target analyte which is monitored by these devices. Herein, we provide a concise overview of high frequency acoustic transducers such as quartz crystal microbalance (QCM), surface acoustic wave (SAW) and film bulk acoustic resonators (FBARs) to compare their working principles, resonance frequencies, selection of piezoelectric materials for their fabrication, temperature-frequency dependency and operation in the liquid phase. The selected sensor applications of these high frequency acoustic transducers are discussed primarily focusing on the two main sensing domains, i.e., biosensing for working in liquids and gas/vapor phase sensing. Furthermore, the sensor performance of high frequency acoustic transducers in selected cases is compared with well-established analytical tools such as liquid chromatography mass spectrometry (LC-MS), gas chromatographic (GC) analysis and enzyme-linked immunosorbent assay (ELISA) methods. Finally, a general comparison of these acoustic devices is conducted to discuss their strengths, limitations, and commercial adaptability thus, to select the most suitable transducer for a particular chemical/biochemical sensing domain.

Label-Free Bioanalyte Detection from Nanometer to Micrometer Dimensions-Molecular Imprinting and QCMs †

Modern diagnostic tools and immunoassay protocols urges direct analyte recognition based on its intrinsic behavior without using any labeling indicator. This not only improves the detection reliability, but also reduces sample preparation time and complexity involved during labeling step. Label-free biosensor devices are capable of monitoring analyte physiochemical properties such as binding sensitivity and selectivity, affinity constants and other dynamics of molecular recognition. The interface of a typical biosensor could range from natural antibodies to synthetic receptors for example molecular imprinted polymers (MIPs). The foremost advantages of using MIPs are their high binding selectivity comparable to natural antibodies, straightforward synthesis in short time, high thermal/chemical stability and compatibility with different transducers. Quartz crystal microbalance (QCM) resonators are leading acoustic devices that are extensively used for mass-sensitive measurements. Highlight features of QCM devices include low cost fabrication, room temperature operation, and most importantly ability to monitor extremely low mass shifts, thus potentially a universal transducer. The combination of MIPs with quartz QCM has turned out as a prominent sensing system for label-free recognition of diverse bioanalytes. In this article, we shall encompass the potential applications of MIP-QCM sensors exclusively label-free recognition of bacteria and virus species as representative micro and nanosized bioanalytes.

Imprinted Oxide and MIP/Oxide Hybrid Nanomaterials for Chemical Sensors †

The oxides of transition, post-transition and rare-earth metals have a long history of robust and fast responsive recognition elements for electronic, optical, and gravimetric devices. A wide range of applications successfully utilized pristine or doped metal oxides and polymer-oxide hybrids as nanostructured recognition elements for the detection of biologically relevant molecules, harmful organic substances, and drugs as well as for the investigative process control applications. An overview of the selected recognition applications of molecularly imprinted sol-gel phases, metal oxides and hybrid nanomaterials composed of molecularly imprinted polymers (MIP) and metal oxides is presented herein. The formation and fabrication processes for imprinted sol-gel layers, metal oxides, MIP-coated oxide nanoparticles and other MIP/oxide nanohybrids are discussed along with their applications in monitoring bioorganic analytes and processes. The sensor characteristics such as dynamic detection range and limit of detection are compared as the performance criterion and the miniaturization and commercialization possibilities are critically discussed.

Molecular Imprinting and Functional Polymers for All Transducers and Applications

The main challenge in developing a chemical sensor is the synthesis of recognition coatings, which are very sensitive and selective to analytes of interest. Molecular imprinting has proven to be the most innovative strategy for this purpose in functional polymer design in the last few decades. Moreover, the introduction of functional groups brings about new applications for all available transducers. Sensitivity and selectivity features of sensor coatings can be tuned by this approach. The strategy produces molecular cavities and interaction sites in sensor coatings. The synthesis of these tailored recognition materials is performed in an outstanding manner, saving time and the high costs of chemicals. Furthermore, intermolecular interactions between the analyte and chemical layers will generate sites that are complementary to the analyte. This procedure can easily be done, directly on a transducer surface, which entails engulfing the analyte by a prepolymer and crosslinking the polymeric material. These imprinted polymers form a robust recognition layer on the transducer surface, which cannot be peeled off and can withstand very harsh conditions, both in gaseous and liquid media. These recognition materials are very suitable, for small molecules and even large bioparticles.

Surface Acoustic Wave (SAW) for Chemical Sensing Applications of Recognition Layers

Surface acoustic wave (SAW) resonators represent some of the most prominent acoustic devices for chemical sensing applications. As their frequency ranges from several hundred MHz to GHz, therefore they can record remarkably diminutive frequency shifts resulting from exceptionally small mass loadings. Their miniaturized design, high thermal stability and possibility of wireless integration make these devices highly competitive. Owing to these special characteristics, they are widely accepted as smart transducers that can be combined with a variety of recognition layers based on host-guest interactions, metal oxide coatings, carbon nanotubes, graphene sheets, functional polymers and biological receptors. As a result of this, there is a broad spectrum of SAW sensors, i.e., having sensing applications ranging from small gas molecules to large bio-analytes or even whole cell structures. This review shall cover from the fundamentals to modern design developments in SAW devices with respect to interfacial receptor coatings for exemplary sensor applications. The related problems and their possible solutions shall also be covered, with a focus on emerging trends and future opportunities for making SAW as established sensing technology.

Graphene Hybrid Materials in Gas Sensing Applications

Graphene, a two dimensional structure of carbon atoms, has been widely used as a material for gas sensing applications because of its large surface area, excellent conductivity, and ease of functionalization. This article reviews the most recent advances in graphene hybrid materials developed for gas sensing applications. In this review, synthetic approaches to fabricate graphene sensors, the nano structures of hybrid materials, and their sensing mechanism are presented. Future perspectives of this rapidly growing field are also discussed.

Mechanistische Untersuchungen zur Ligandenfluktuation in den Sandwich-Komplexen [M([12]Krone-4)2]++ / Mechanistic Studies on the Ligand Fluctuations in the Sandwich Complexes [M([12]crown-4)2]++

The metal ions Co++ , Ni++ , Mg++ , and Zn++ form the sandwich complexes [M([12]crown-4)2]++ with [12]crown-4 in CD3NO2. From the reaction order of the crown ether exchange a strong outer sphere association between complex and ligand can be inferred. Ligand fluctuation processes occur via an intramolecular associative mechanism from which a very high symmetry of the complexes results.

[M(II)([15]Krone-5)(L)2 ]++ -Komplexe -der Einfluß des Kronenetherhohlraums auf die Mobilität der Liganden L / [M(II)([15]Crown-5)(L)2]++ Complexes - The Influence of the Crown Ether Cavity on the Mobility of the Ligands L

The mobility of coordinated methanol in the complex [Co(II)(Benzo[15]crown-5)(CH3OH)2]f + was studied in nitromethane. The methanol exchange mechanism tends to an associative nature with an increasing amount of methanol in the solutions as can be deduced from the activation entropy. All activation data can be combined in an isokinetic relationship which is also valid for the respective complex with the unsubstituted [15]crown-5 ether ligand

1H-DNMR-Untersuchungen zur inter- und intramolekularen Ligandenmobilität im System Co(II)/[18]Krone-6 / 1H DNMR Studies on the Inter- and Intramolecular Ligand Mobility in the System Co(II)/[18]crown-6

The crown ether exchange could be studied in non-coordinating solvents by using the hy­drophobic complex [Co([18]crown-6)] + + . Appreciable outer-sphere association between complex and ligand (K°(50 °C) = 12 ± 0.5 M-1) occurs. This changes the second order rate law to first order (kmono(50 °C) = 7900 ± 500 s_1) with increasing ligand concentration. In the complex mer-[Co([18]crown-6)(CH3OH)3]+ + the stereochemical rearrangement of the uncoordinated part of the crown ether follows an intramolecular pathway via the complex [Co([18]crown-6)]++ as an intermediate product.

Conductometric sensors for monitoring degradation of automotive engine oil

Conductometric sensors have been fabricated by applying imprinted polymers as receptors for monitoring engine oil quality. Titania and silica layers are synthesized via the sol-gel technique and used as recognition materials for acidic components present in used lubricating oil. Thin-film gold electrodes forming an interdigitated structure are used as transducers to measure the conductance of polymer coatings. Optimization of layer composition is carried out by varying the precursors, e.g., dimethylaminopropyltrimethoxysilane (DMAPTMS), and aminopropyl-triethoxysilane (APTES). Characterization of these sensitive materials is performed by testing against oil oxidation products, e.g., carbonic acids. The results depict that imprinted aminopropyltriethoxysilane (APTES) polymer is a promising candidate for detecting the age of used lubricating oil. In the next strategy, polyurethane-nanotubes composite as sensitive material is synthesized, producing appreciable differentiation pattern between fresh and used oils at elevated temperature with enhanced sensitivity.

A Mass-Sensitive Approach for the Detection of Anaesthetic Xenon

Quartz crystal microbalances (QCMs) were utilized for the detection of the noble gas xenon (Xe) by combining them with different recognition layers such as permethylated calixarenes (tetramethyl-tert-butylcalix[4]arene (Cal4Me), hexamethyl-tert-butylcalix[6]arene (Cal6Me)), and polyurethanes, with covalently embedded Cal4OH (Poly4Cal), or Cal6OH (Poly6Cal). A third type of sensitive material is synthesized from polyacrylic acid (PAA) and polyvinyl alcohol (PVA) and utilized as a sensitive coating. The results demonstrate that the Cal4Me layer gives higher response towards Xe, while, by the use of a second uncoated QCM channel, the influence of ambient humidity could be nearly completely compensated by signal subtraction. Moreover, the Cal4Me sensor shows excellent reversibility and rapid response time, providing a potentially reliable way to determine Xe during anaesthesia.

Functionalized Polymers as Receptors for Detection of Cells

Mass sensitive sensors were applied for fast and label-free detection of bio-analytes. Robust and miniaturized sensor devices were fabricated by combining bio-mimetic imprinted surfaces with quartz crystal microbalances for the analysis of yeast and bacteria cells. These sensors allow us to differentiate between different growing stages of yeast cells. Moreover, the viability of cells was detected by structuring quartz crystal microbalance electrodes like a grid. Artificial yeast cells were produced to pattern the recognition layer, giving reversible enrichment of the respective bio-analytes. This approach was followed to ensure the reproducibility of the identical sensitive material in each case, because the properties of each cell depend on its growth stage, which varies over time. The strategy was further applied to develop a sensitive system for Escherichia coli. Structuring of these materials by soft lithography allows differentiation between cell strains, e.g. E. coli (strain W & B) with a five-fold selectivity.

QCM-arrays for sensing terpenes in fresh and dried herbs via bio-mimetic MIP layers

A piezoelectric 10 MHz multichannel quartz crystal microbalance (MQCM), coated with six molecularly imprinted polystyrene artificial recognition membranes have been developed for selective quantification of terpenes emanated from fresh and dried Lamiaceae family species, i.e., rosemary (Rosmarinus Officinalis L.), basil (Ocimum Basilicum) and sage (Salvia Officinalis). Optimal e-nose parameters, such as layer heights (1-6 KHz), sensitivity <20 ppm of analytes, selectivity at 50 ppm of terpenes, repeatability and reproducibility were thoroughly adjusted prior to online monitoring. Linearity in reversible responses over a wide concentration range <20-250 ppm has been achieved. Discrimination between molecules of similar molar masses, even for isomers, e.g. α-pinene and β-pinene is possible. The array has proven its sensitive and selective properties of sensor responses (20-1,200 Hz) for the difference of fresh and dried herbs. The sensor data attained was validated by GC-MS, to analyze the profiles of sensor emanation patterns. The shelf-life of herbs was monitored via emanation of organic volatiles during a few days. Such an array in association with data analysis tools can be utilized for characterizing complex mixtures.

Solvent vapour detection with cholesteric liquid crystals--optical and mass-sensitive evaluation of the sensor mechanism

Cholesteric liquid crystals (CLCs) are used as sensitive coatings for the detection of organic solvent vapours for both polar and non-polar substances. The incorporation of different analyte vapours in the CLC layers disturbs the pitch length which changes the optical properties, i.e., shifting the absorption band. The engulfing of CLCs around non-polar solvent vapours such as tetrahedrofuran (THF), chloroform and tetrachloroethylene is favoured in comparison to polar ones, i.e., methanol and ethanol. Increasing solvent vapour concentrations shift the absorbance maximum to smaller wavelengths, e.g., as observed for THF. Additionally, CLCs have been coated on acoustic devices such as the quartz crystal microbalance (QCM) to measure the frequency shift of analyte samples at similar concentration levels. The mass effect for tetrachloroethylene was about six times higher than chloroform. Thus, optical response can be correlated with intercalation in accordance to mass detection. The mechanical stability was gained by combining CLCs with imprinted polymers. Therefore, pre-concentration of solvent vapours was performed leading to an additional selectivity.

Detection of viruses with molecularly imprinted polymers integrated on a microfluidic biochip using contact-less dielectric microsensors

Rapid detection of viral contamination remains a pressing issue in various fields related to human health including clinical diagnostics, the monitoring of food-borne pathogens, the detection of biological warfare agents as well as in viral clearance studies for biopharmaceutical products. The majority of currently available assays for virus detection are expensive, time-consuming, and labor-intensive. In the present work we report the creation of a novel micro total analysis system (microTAS) capable of continuously monitoring viral contamination with high sensitivity and selectivity. The specific interaction between shape and surface chemistry between molecular imprinted polymer (MIP) and virus resulted in the elimination of non-specific interaction in the present sensor configuration. The additional integration of the blank (non-imprinted) polymer further allowed for the identification of non-specific adsorption events. The novel combination of microfluidics containing integrated native polymer and MIP with contact-less dielectric microsensors is evaluated using the Tobacco Mosaic Virus (TMV) and the Human Rhinovirus serotype 2 (HRV2). Results show that viral binding and dissociation events can be readily detected using contact-less bioimpedance spectroscopy optimized for specific frequencies. In the present study optimum sensor performance was achieved at 203 kHz within the applied frequency range of 5-500 kHz. Complete removal of the virus from the MIP and device reusability is successfully demonstrated following a 50-fold increase in fluid velocity. Evaluation of the microfluidic biochip revealed that microchip technology is ideally suited to detect a broader range of viral contaminations with high sensitivity by selectively adjusting microfluidic conditions, sensor geometries and choice of MIP polymeric material.

Synthetic receptors for selectively detecting erythrocyte ABO subgroups

Surface imprinting techniques with erythrocytes as templates yield polymer coatings with selective recognition sites towards red blood cells. The resulting cavities in the respective surface exhibit selectivity between blood subgroups as shown by Quartz Crystal Microbalance (QCM) measurements. Mass sensitive effects in the kilohertz range could be observed for concentrations down to 0.5 x 10(8) cells/mL. Frequency response as well as recovery of the sensor took place within a few minutes, indicating that no covalent binding is involved. Linear concentration dependence over a defined region provides ideal conditions for cross selectivity measurements. A1 imprinted sensor coatings resulted in an effect of 40 kHz when exposed to the template blood group, while A2 erythrocytes yielded just 11% of that value on the same layer. Furthermore, A2 imprinted coatings incorporated only one third the amount of A1 erythrocytes as compared to A2 ones. Therefore, imprinted materials depict the entire cell surface and utilize it for recognition, whereas natural antibodies bind on the defined antigen position and thus usually cannot distinguish between cells carrying different amounts of them.

Real-life application of a QCM-based e-nose: quantitative characterization of different plant-degradation processes

Continuous surveillance of composting processes would enable a feedback loop to be obtained for both analysis and process control. For this purpose, we designed e-noses based on a six-electrode quartz-crystal microbalance (QCM) array coated with affinity materials and molecularly imprinted polymers (MIP). They enable quantitative monitoring of volatile organic compounds (VOCs) emitted directly in a compost bin and are highly suitable tools for achieving on-line characterization of the degradation processes occurring. During grass and pine composting (duration 14 days and 40 days, respectively), we observed concentrations of up to 250 ppm of esters, 700 ppm of alcohols, 250 ppm of terpenes, and 90% relative humidity directly on-line with such a system and could validate the data off-line by GC-MS. The sensor also gave direct insight into the differences between the two composting batch types. Besides duration, during grass composting larger amounts of alcohols are emitted whereas relative amount of terpenes is twice as high for pine composting. Detailed correlation of the sensor and the GC-MS data allows approximate estimation of the sensitivity of the sensor materials towards analyte classes such as, e.g., aliphatic alcohols or terpenes.

Molecularly imprinted sol–gel nanoparticles for mass-sensitive engine oil degradation sensing

Titanate sol-gel layers imprinted with midchain carbonic acids have proven highly useful for detecting engine oil degradation processes owing to selective incorporation of oxidised base oil components. Synthesising the material from TiCl(4) in CCl(4) and precipitating with water leads to imprinted TiO(2) nanoparticles with a diameter of 200-300 nm. Replacing the water by a 1 M ammonium hydroxide solution reduces the average particle size to 50-100 nm with retention of the interaction capabilities. Experiments with the latter solution revealed that the 100-nm particles take up substantially more analyte, indicating a size-dependent phenomenon. As the number of interaction sites within each material is the same, this cannot be a consequence of thermodynamics but must be one of accessibility. The sensor characteristic of water-precipitated particles towards engine oil degradation products shows substantially increased sensitivity and dynamic range compared with the corresponding thin films. Coating quartz crystal microbalances with such nanoparticle materials leads to engine oil degradation sensors owing to incorporation of acidic base oil oxidation products. Interaction studies over a large range of layer thicknesses revealed that both the absolute signal and the steepness of the correlation between the sensor signal and the layer height is 2 times higher for the particles.

Sensor technology and its application in environmental analysis

Environmental analysis is one of the fundamental applications of chemical sensors. In this review we describe different sensor systems for the gas and liquid phases that have been tested either with real-life samples or in the field during the last five years. Most field sensors rely either on electrochemical or optical transducers. In the gas phase, systems have been proposed for analysis of oxides of nitrogen, carbon, and sulfur in air, and volatile organic compounds. In the liquid phase, most detection systems used for real-life samples detect heavy-metal ions or organic contamination, for example pesticides, organic solvents and polycyclic aromatic hydrocarbons.

Cavities generated by self-organised monolayers as sensitive coatings for surface acoustic wave resonators

Silanisation of quartz substrate surfaces with a mixture of two chlorosilanes, namely trimethylchlorosilane and 7-octenyldimethylchlorosilane, leads to sensitive coatings for volatile organic compounds (VOC) on surface acoustic wave (SAW) devices. In this way we created monolayers of molecular cavities engulfing the analytes according to host-guest chemistry directly on the device surfaces, and also confirmed the occurrence of such cavities by molecular modelling. We monitored the binding process of the silanes by using Fourier transform infrared (FTIR) spectrometry and atomic force microscopy (AFM). In order to increase the stiffness of the cavities, we crosslinked the terminal double bonds of the long spacers by heating the surface in the presence of a radical initiator. Compared to SAW delay lines silanised with trimethylchlorosilane, devices modified with the binary silane mixture lead to substantially higher frequency shifts when exposed to solvent vapour streams. Nearly instantaneous responses can be observed, which e.g. allows xylene detection down to a few ppm.

Surface imprinting strategies for the detection of trypsin

Self-organized receptor layers are synthesized by molecular imprinting methods directly on pre-coated 10 MHz quartz-crystal microbalances (QCMs). The surface-imprinting is performed by three methods using amorphous, crystalline and solubilized trypsin, respectively, as templates. These attempts allowed us to compare imprinting results obtained with templating proteins in the dry state as well as in aqueous solution. All methods are generally applicable for surface imprinting of thin films. The biomimetic sensor layers allow selective enzyme enrichment on the imprinted electrode with detection limits as low as 100 ng ml(-1) and response times of a few minutes. Solution-based polymer imprinting with native trypsin as template resulted in the highest specific enzyme recognition, which even allowed us to distinguish denatured trypsin from the native form.

Synthetic receptors for chemical sensors—subnano- and micrometre patterning by imprinting techniques

Antibody-like selectivities are introduced into man-made polymeric systems by means of molecular imprinting. On the molecular scale, organic contaminants in water were analysed by 10 MHz QCM sensors coated both with affinity layers and with selective imprinted polymers. Affinity layers, polystyrene/silicone, yield sensor effects for aromatic hydrocarbons according to, e.g., molecular weight and solubilities, a prediction by molecular modelling is proposed. Imprinted layers, however, recognise molecular shapes: the lean toluene is favoured by factor of six to the more bulky o-xylene; even the three xylenes can be distinguished from each other. On the micrometre scale, erythrocytes of different blood groups are differentiated, although their geometrical shape is identical. In this case, the recognition is accomplished by a predefined hydrogen bonding network between excess OH groups of the polyurethane layer and the sugar molecules on the cell surface defining the blood group.

Bioimprinted QCM sensors for virus detection-screening of plant sap

Surface imprinting techniques on polymer-coated quartz-crystal microbalances (QCM) have been used to detect tobacco mosaic viruses (TMV) in aqueous media. Molecularly imprinted polymers (MIP), tailor-made by self organisation of monomers around a template (TMV), were generated directly on the gold electrodes. Imprinted trenches on the polymer surface mimicking the shape and surface functionality of the virus serve as recognition sites for re-adsorption after washing out of the template. The sensors are applicable to TMV detection ranging from 100 ng mL(-1) to 1 mg mL(-1) within minutes. Furthermore, direct measurements without time-consuming sample preparation are possible in complex matrices such as tobacco plant sap.

QCM array for on-line-monitoring of composting procedures

Six QCM resonators forming a sensor array were coated with different molecularly imprinted polymers for the on-line monitoring of composting procedures. Four key analytes are traced, namely water, 1-propanol, ethyl acetate and limonene. Trendlines obtained on-line by the sensor during measurements in a commercial composter give a distinct pattern: the signal for the alcohols first decreases according to an increase in ethyl acetate concentration, and increases again, when obviously no more acetic acid is formed. Limonene is detected in later stages of composting. Similar trends could also be observed by GC-MS. Additionally, chromatographic and sensor data for limonene could be correlated with each other.

Sensor strategies for microorganism detection--from physical principles to imprinting procedures

Detecting cells and microorganisms in different matrices is becoming an increasingly important task in a variety of fields including bioprocess control, food technology, health care, and environmental analysis. In this review, fast on-line detection methods for this purpose are presented including different recognition and transducer strategies.