Optical colorimetric sensor arrays for chemical and biological analysis

In recent years, the sensor array has attracted much attention in the field of complex system analysis on the basis of its good selectivity and easy operation. Many optical colorimetric sensor arrays are designed to analyze multi-target analytes due to the good sensitivity of optical signal. In this review, we introduce the targeting analytes, sensing mechanisms and data processing methods of the optical colorimetric sensor array based on optical probes (including organic molecular probes, polymer materials and nanomaterials). The research progress in the detection of metal ions, anions, toxic gases, organic compounds, biomolecules and living organisms (such as DNA, amino acids, proteins, microbes and cells) and actual sample mixtures are summarized here. The review illustrates the types, application advantages and development prospects of the optical colorimetric sensor array to help broad readers to understand the research progress in the application of chemical sensor array.     

Evaluation of doped and undoped poly (o‐anisidine) as sensing materials for a sensor array for volatile organic compounds

Poly (o‐anisidine) (PoANI) and PoANI doped with nickel oxide and zinc oxide were evaluated as sensing materials for four gas analytes (methanol, ethanol, acetone, and benzene). The sensing materials had high sensitivity (showing an affinity towards the target analytes even at low concentrations, in the range of 1‐5 ppm), but rather poor selectivity, especially when the gas analytes were in a mixture. To exploit the poor selectivity, the three sensing materials were combined into a sensor array using principal component analysis (PCA) as a sensing algorithm. It was found that using a sensor array, the four individual gases could be separated. However, when all four gases were present (in analyte mixtures), there was too much overlap in the responses to distinguish between individual gas analytes and their related mixtures.     

Molecularly Imprinted Photonic Polymers as Sensing Elements for the Creation of Cross‐Reactive Sensor Arrays

By combining molecular imprinting and colloidal crystal templating, molecularly imprinted inverse‐opal photonic polymers (MIPPs) acting as sensing elements have been exploited to create sensor arrays for the first time. With this new strategy, abundant sensing elements with differential sensing abilities were easily accessible. Because of the unique hierarchical porous structure integrated in each sensing element, high sensitivity and selectivity, fast response and self‐reporting (label‐free) detection could be simultaneously achieved. All these fascinating features indicate that MIPPs are ideal sensing elements for creating sensor arrays. By integrating the individual sensing elements on a substrate, the formed array chip delivers better portability and high‐throughput capability. As a demonstration, six kinds of contaminants were selected as analytes. The detection and discrimination of these analytes and even their mixtures in a wide range of concentrations, particularly trace amounts of analyte against a high background of other components, could be achieved, indicating the powerful capability of MIPPs‐based sensor array for sensing. These results suggest that the described strategy opens a new route for sensor array creation and should find important applications in a wide range of areas.     

Nano‐Graphene Oxide Based Multichannel Sensor Arrays towards Sensing of Protein Mixtures

Optical array‐based sensors are attractive candidates for the detection of various bio‐analytes due to their convenient fabrication and measurements. For array‐based sensors, multichannel arrays are more advantageous and used frequently in many electronic sensors. But most reported optically array based sensors are constructed on a single channel array. This difficulty is mainly instigated from the overlap in optical responses. In this report we have used nano‐graphene oxide (nGO) and suitable fluorophores as sensor elements to construct a multichannel sensor array for the detection of protein analytes. By using the optimized multichannel array we are able to detect different proteins and mixtures of proteins with 100 % classification accuracy at sub‐nanomolar concentration. This modified method expedites the sensing analysis as well as minimizes the use of both analyte and sensor elements in array‐based protein sensing. We have also used this system for the single channel array‐based sensing to compare the sensitivity and the efficacy of these two systems for other applications. This work demonstrated an intrinsic trade‐off associated with these two methods which may be necessary to balance for array‐based analyte detections.     

A fluorescent anion sensor that works in neutral aqueous solution for bioanalytical application

Anion recognition and anion sensing are of interest because anions play many important roles in living organisms. Most currently known anion sensors work only in organic solution, but sensors for biological applications are required to function in neutral aqueous solution. We have designed and synthesized a novel fluorescent sensor for anions. The sensor molecule 1-Cd(II) contains 7-amino-4-trifluoromethylcoumarin as a fluorescent reporter and Cd(II)-cyclen (1,4,7,10-tetraazacyclododecane) as an anion host. In neutral aqueous solution, Cd(II) of 1-Cd(II) is coordinated by the four nitrogen atoms of cyclen and the aromatic amino group of coumarin. When various anions are added to 100 mM HEPES buffer solution (pH 7.4) containing 1-Cd(II), the aromatic amino group of coumarin is displaced from Cd(II), causing a change of the excitation spectrum. While pyrophosphate and citrate were detected with high sensitivity, fluoride and perchlorate produced no response. Among organic anions, ATP and ADP gave strong signals, while cAMP showed little signal. By utilizing the different affinities of the sensor for AMP and cAMP, the activity of phosphodiesterase, which cleaves cyclic nucleotide, was monitored in real-time. The sensor should have many biochemical and analytical applications and the sensing principle should be widely applicable to the sensing of other molecules.     

Schiff base derived from salicylaldehyde‐based azo dye as chromogenic anionic sensor and specific turn‐on emission sensor for cyanide ion

A novel Schiff base has been derived from condensing 4‐aminoantipyrine with diazotized salicylaldehyde. The derived compound acted as a colorimetric sensor for hazardous aqueous anions like CN^?, F^?, and CH₃COO^? among a list of anions. The colorimetric changes were further verified through absorption titrations. The detection limits were of the order of 10^?10 M, which makes the sensor significant. The interaction of the anions with the sensor was stoichiometrically 1:1 with good binding constants. The sensor turns out to be a specific turn‐on emission sensor for CN^? even in competitive environments. The F^? ion sensing ability was extended to the determination of F^? in a commercial toothpaste with good results.     

Colorimetric molecularly imprinted polymer sensor array using dye displacement

A colorimetric sensor array composed of seven molecularly imprinted polymers was shown to accurately identify seven different aromatic amines. The response patterns were systematically classified using linear discriminant analysis with 94% classification accuracy. Analyses of the response patterns of the analytes to the imprinted polymer array suggest that the different selectivity patterns, although subtle, appear to arise from the imprinting process. The molecular imprinting process enabled the rapid preparation of the polymers in the array from ethylene glycol dimethacrylate and methacrylic acid (80:20) in the presence of six different template molecules plus a blank nonimprinted polymer. The response of the imprinted polymer array was coupled to a colorimetric response, using a dye displacement strategy. A benzofurazan dye was selected and shown to give an accurate measure of the binding properties of the imprinted polymer array to all seven analytes. The colorimetric response also enabled the inclusion of analytes that are not spectroscopically active and were not among the original analytes that were used as template molecules. This broadens the potential utility of the imprinted polymer sensor array strategy to a wider range of analytes and applications.     

A simple and efficient colorimetric anion sensor based on a thiourea group in DMSO and DMSO-water and its real-life application

An efficient colorimetric sensor with a thiourea moiety as binding sites and p-nitrophenylhydrazine as a signaling unit has been synthesized by only one single-step procedure. Selectivity for anions with the distinct geometry (tetrahedral, trigonal planar and spherical) has been investigated in dry DMSO and even in DMSO/H(2)O (95:5, v/v) solutions through the naked-eye experiment, UV-vis titration and (1)H NMR titration techniques. In particular, the fluoride of toothpaste can be detected qualitatively by the sensor 1.     

Functionalized gold nanoparticle supported sensory mechanisms applied in detection of chemical and biological threat agents: A review

There is a great necessity for development of novel sensory concepts supportive of smart sensing capabilities in defense and homeland security applications for detection of chemical and biological threat agents. A smart sensor is a detection device that can exhibit important features such as speed, sensitivity, selectivity, portability, and more importantly, simplicity in identifying a target analyte. Emerging nanomaterial based sensors, particularly those developed by utilizing functionalized gold nanoparticles (GNPs) as a sensing component potentially offer many desirable features needed for threat agent detection. The sensitiveness of physical properties expressed by GNPs, e.g. color, surface plasmon resonance, electrical conductivity and binding affinity are significantly enhanced when they are subjected to functionalization with an appropriate metal, organic or biomolecular functional groups. This sensitive nature of functionalized GNPs can be potentially exploited in the design of threat agent detection devices with smart sensing capabilities. In the presence of a target analyte (i.e., a chemical or biological threat agent) a change proportional to concentration of the analyte is observed, which can be measured either by colorimetric, fluorimetric, electrochemical or spectroscopic means. This article provides a review of how functionally modified gold colloids are applied in the detection of a broad range of threat agents, including radioactive substances, explosive compounds, chemical warfare agents, biotoxins, and biothreat pathogens through any of the four sensory means mentioned previously.     

Study of ionic liquid immobilization on polyvinyl ferrocene substrates for gas sensor arrays

In this report, the effects of conductive polymer oxidation states and structures on the design and development of ionic liquid (IL)/conductive polymer (CP) composite films for gas sensing are systematically characterized. Four different polyvinyl ferrocene (PVF) films synthesized by varying the conditioning potential (0.7 vs 0.0 V) and the electrolyte are tested for their gas-sensing properties (e.g., sensitivity, selectivity, response time, linearity, and dynamic range against various gas analytes such as dichloromethane, ethanol, natural gas, methane, formaldehyde (37%), and benzene) utilizing the quartz crystal microbalance (QCM) and ATR-FT-IR. The best available film is further studied as a substrate for the immobilization of various ILs that enhanced both the sensitivity and selectivity. Finally, two arrays, each comprising four sensors with the following scheme are developed and characterized for their ability to classify the four target analytes by using linear discriminant analysis: (1) the highest sensitivity PVF film immobilized with four different ILs and (2) the highest sensitivity IL immobilized in four different PVF films. Array 2 is proven to be much better than array 1 in discriminating the analytes, which is very significant in establishing the fact that a diverse set of PVF redox states allow the rational development of a PVF/IL composite-based sensor array in order to analyze complex mixtures utilizing structural differences and the extent of intermolecular interactions.     

Determination of glycerophosphate and other anions in dentifrices by ion chromatography

Simple, reliable and sensitive analytical methods to determine the anions, such as fluoride, monofluorophaosphate, glycerophosphate related to anticaries are necessary for basic investigations of anticaries and quality control of dentifrices. A method for the simultaneous determination of organic acids, organic anions and inorganic anions in the sample of commercial toothpaste is proposed. Nine anions (fluoride, chloride, nitrite, nitrate, sulfate, phosphate, monofluorophaosphate, glycerophosphate and oxalic acid) were analyzed by means of ion chromatography using a gradient elution with KOH as mobile phase, IonPac AS18 as the separation column and suppressed conductivity detection. Optimized analytical conditions were further validated in terms of accuracy, precision and total uncertainty and the results showed the reliability of the IC method. The relative standard deviations (RSD) of the retention time and peak area of all species were less than 0.170 and 1.800%, respectively. The correlation coefficients for target analytes ranged from 0.9985 to 0.9996. The detection limit (signal to noise ratio of 3:1) of this method was at low ppb level (<15 ppb). The spiked recoveries for the anions were 96-103%. The method was applied to toothpaste without interferences.     

An approach to the spectral simulation of infrared hollow waveguide gas sensors

Optical simulations enable to model an entire chemical gas sensing platform based on hollow waveguides (HWGs) operating in the mid-infrared spectral regime using a three-dimensional representation of the sensor components and taking the spectral response to virtual analytes into account. Furthermore, a strategy for including the spectral response of dielectrically coated HWGs is demonstrated. Utilizing experimentally obtained spectroscopic data recorded at well-defined conditions, the complex refractive indices of selected target analytes (i.e., methane, butane, and isobutylene) have been derived based on a refined harmonic oscillator model. In turn, these parameters have enabled to directly assign the dielectric functions of these analytes to virtual objects representing the analyte within the modeled sensor setup. In a next step, spectroscopic sensor response functions have been simulated as absorbance spectra across selected wavelength regimes utilizing spectrally resolved ray-tracing techniques and have been compared to experimental data.     

Preparation of cotton fabric based non-invasive colorimetric sensor for instant detection of ketones

Cotton based colorimetric sensing offers great potential for the detection of desired analytes due to their easy naked-eye sensing, ease in fabrication, cost effectiveness, rapid recognition, high sensitivity and selectivity. Poly (2-(dimethylamino ethyl) methacrylate) (pDMAEMA) and sodium nitroprusside (SNP) were deposited alternatively on a treated cotton fabric for non invasive calorimetric sensing of ketones. Physical and chemical characterizations such as FESEM, EDS, XRD, UV–vis, FTIR and XPS of the treated cotton based sensor were carried out. Standard colour chart of the targeted ketones is prepared by dissolving appropriate amount of them with distilled water to get down to the final concentration as 80 mM and 10 µL of each solution of analyte was dropped on the surface of treated cotton fabric. The fabricated sensor is also tested for its practicability by analyzing the targeted ketones in human urine and the results obtained were compared with the standard colour chart. The results shows consistency in colours thereby making it a quick, efficient, eco-friendly and reliable monitoring tool to detect various ketones.     

A reversible dual-channel chemosensor for fluoride anion

A colorimetric and fluorescent fluoride probe bearing phenolic hydroxy and imine groups has been designed and synthesised. This receptor could visually and spectroscopically recognise F^?  with high selectivity over other anions. After the addition of fluoride ions to the solution of ([1,1′-biphenyl]-4,4′-diylbis (azanylylidene)) bis (methanylylidene)) bis (naphthalen-2-ol) (TY), since the deprotonation reaction occurred between the sensor and fluoride, the fluorescence intensity of the solution changed significantly. Furthermore, the quenched fluorescence caused by fluoride ions could be recovered upon the addition of calcium ions to this complex solution. This resulted in an ‘OFF-ON-OFF’ type sensing. In particular, an IMP logic gate has been proposed using the output obtained from the fluorescence studies. The fluorescence, UV-vis titration and ¹H NMR titration experiments indicated that the effects might occur via a combined process including hydrogen bond and deprotonation between the sensor and F^? .     

Differential receptors create patterns diagnostic for ATP and GTP

Herein we report the combination of a library of resin-bound sensors along with a multicomponent sensor array. This novel combinatorial array sensor system shows selectivity for nucleotide phosphates in solution. The design of the anchored receptor includes a 1,3,5-trisubstituted-2,4,6-triethylbenzene scaffold coupled with peptide libraries. Each chemosensor is placed into a micromachined cavity within a silicon wafer, and the optical changes observed by a charged-coupled device result in near-real-time digital analysis of solutions. A colorimetric displacement assay was performed, and time-dependent imaging studies of the selected sensing ensembles result in a differential responses upon addition of adenosine 5'-triphosphate (ATP), adenosine 5'-monophosphate (AMP), or guanosine 5'-triphosphate (GTP). An advantage to this approach is that it creates an array of sensors that gives a fingerprint response for each analyte. Principal component analysis indicates that the library of chemosensors can differentiate between ATP, GTP, and AMP. On the basis of factor loading values, individual sensors from the library were sequenced to elucidate their chemical composition.     

A colorimetric sensor array for organics in water

Molecular recognition of organic compounds in aqueous solutions is inherently challenging due to the potential interference from the very high concentration of water. Here we present a simple colorimetric sensor array that probes a wide range of chemical properties. By printing hydrophobic dyes on a hydrophobic membrane, sensor arrays are easily prepared that provide substantial chemical selectivity for the identification and quantification of various organics (both single compounds and complex mixtures) dissolved in water. It is possible to differentiate easily even among closely related organic compounds. Upon immersion in aqueous solutions, digital imaging of the dye array before and after exposure to an analyte provide a color change profile that is a fingerprint for the organic components of the solution. Facile identification of a wide variety of aqueous organic solutions is possible over a concentration range of 0.1 M to 1 muM. Complex mixtures present no inherent difficulty; for example, a series of commercial soft drinks were easily distinguished using the colorimetric sensor array approach.     

Photophysical and anion sensing properties of platinum(II) terpyridyl complexes with phenolic ethynyl ligands

A series of platinum(ii) terpyridyl complexes with phenolic ethynyl ligands have been synthesized and characterized. Their photophysical and sensing properties towards anions such as fluoride, acetate and dihydrogenphosphate have been investigated. These complexes show a colorimetric response and fluorescence quenching in the presence of anions including fluoride, acetate and dihydrogenphosphate, and selective sensing towards fluoride in some cases. The sensing mechanism has been investigated by spectrophotometric and (1)H NMR titration.     

Colorimetric Fluoride‐Anion Sensor Based on Intramolecular Hydrogen Bonding and Enol–Keto Tautomerization of a Phenothiazine Derivative

A colorimetric sensor for fluoride ions based on a new sensing mechanism is reported. The colorimetric sensor contains an isomerizable enol–keto moiety as the recognition site and phenothiazine as chromogenic center. A color change visible to the naked eye is observed upon addition of fluoride ions to the solution of sensor 1 in aprotic solvents such as CHCl₃ and MeCN. The sensor shows no colorimetric response for other halide ions. Enol‐keto tautomerization is proposed to be responsible for the anion sensing of 1 , based on UV/VIS absorption, ¹H‐NMR, and single‐crystal structure analysis.     

A novel electrosynthesized polymer applied to molecular imprinting technology

Poly(2-macaptobenzimidazole) (PMBI) films are prepared at the gold electrode surface by electropolymerization using imprinting technology and the target analyte cholesterol is used as the template. A cholesterol-selective sensor based on PMBI film was employed in conjunction with differential pulse voltammetry (DPV) and ferricyanide as mediator. Concentration of cholesterol up to 100 μM could be detected with a linear determination range up to 20 μM and a detection limit of 0.7 μM. The molecular imprinting approach offers a relatively nice selectivity for the sensor toward cholesterol with respect to common coexisting substances. The method is simple and the stability of the electrode prepared is satisfactory. The results of this research show the feasibility of using molecular imprinting methodology for preparing sensing devices for analytes that are electrochemically inactive.     

Selective Autonomous Molecular Transport and Collection by Hydrogel‐Embedded Supramolecular Chemical Gradients

Selective transport and concentration of molecules to specified regions on a substrate both enhances the potential to detect such molecules and provides a path to spatially localize such molecules prior to initiation of subsequent chemical reactions. Here, we first embed radially symmetric α‐, β‐, and γ‐cyclodextrin gradients in a hydrogel matrix. Driven by host‐guest interactions between the cyclodextrins and the target molecule, we observe these gradients can serve to direct 2D molecular transport. Using xanthene dyes and organophosphates as target molecules, we found the transport metrics, e.g., selectivity, rate, and concentration limits, are strongly dependent on the specific cyclodextrin forming the gradient. In all cases, as the concentrating power of the gradient increased, the rate of target concentration slowed, which we hypothesize is because stronger interactions between the target and the cyclodextrin decrease the rate of target diffusion. The concentration enhancement for the nerve agent simulant 4‐methylumbelliferyl phosphate (15.8) is the greatest when the gradient is formed using β‐cyclodextrin while directed concentration of cyanomethyl phosphonate, a smaller non‐aromatic organophosphate, is observed only for the smaller α‐CD. To provide a near real‐time read‐out of the concentration of the analyte, we used an array of IR resonant metallic nanoantennas tuned to a specific IR absorption band of the analyte to enhance the IR signal generated by the analyte.