Biochemical sensors: The state of the art

The basic components of a (bio)chemical sensor and the main concepts involved in the (bio)chemical sensor methodology are considered in order to depict the state of the art of the development of research in this field, paying special attention to the evolution of the published scientific literature in analytical chemistry.     

Fluorescent photoionic devices with two receptors and two switching mechanisms: applications to pH sensors and implications for metal ion detection

Two leading designs of fluorescent sensors are combined to yield the novel hybrid system of the ‘Fluorophore-Receptor₁-Spacer-Receptor₂’ format. We use 4-(dialkylaminoalkylamino)-7-nitrobenzo-2-oxa-1,3-diazoles as examples. The emission from internal charge transfer excited states in the present instances are highly responsive to N-H deprotonation as well as being quenched by intramolecular tertiary amine groups via photoinduced electron transfer (PET). When applied to pH sensing, this leads in favourable cases to two steps in the fluorescence-pH profile which can be viewed as a multi-stable photoionic device, even though single steps are more usual. The former situation is favoured when the two proton-associated equilibria are sufficiently separated on the pH scale and when the PET process is of moderate efficiency. These systems have the added feature of excitation/emission wavelengths in the visible region. As a secondary theme, we point out that caution is required when designing sensors for transition metal ions from systems with intrinsically proton-sensitive fluorescence due to receptors either integrated with or spaced from the fluorophore.     

Chemically Sensitive Field‐Effect Transistors and Chemiresistors: New Materials and Device Structures

Electronic sensor technology remains of widespread and intense interest. There are compelling needs to detect chemical species ranging from small molecules dispersed in the gas phase to complex biopolymers in aqueous solution. This review describes some recent advances in three main areas: chemically sensitive resistors (chemiresistors, CRs) including inorganic and organic based devices, field effect transistors (FETs) with semiconducting layers and/or gates with chemical sensitivity, and sensors based on the differential conductivity of nanotubes and nanowires. Results reported in the last two to three years are emphasized, highlighting some current trends in the development of sensors for applications such as diagnostics, process monitoring, and security.     

Design of a zinc(II) ion specific fluorescence sensor

We report the synthesis of {[3-(biscarboxymethylamino)-2-methoxy-5-methylphenyl]carboxymethylamino}acetic acid, which functions as a Zn²⁺ selective fluorescence probe (sensor).     

A Zn(II) ion selective fluorescence sensor that is not affected by Cd(II)

Comparison of sensors sodium-2,6-diamino-(N,N,N′,N′-tetraacetate)-4-methylanisole 1 and sodium-2,6-diamino-(N,N,N′-triacetate)-4-methylanisole 2 reveal that the loss of an acetyl group in 2 leads to a more selective Zn(II) induced fluorescence enhancement and shows no response to any other metal ions including Cd(II). Structural modifications and AM1 calculations indicate that the sensor uses the three acetyl groups and the 3° amino nitrogen for binding the metal ion. AM1 calculations imply a trigonal bipyramidal coordination for Zn(II) with the solvent molecule occupying one of the axial positions.     

Trends and challenges of refractometric nanoplasmonic biosensors: A review

Motivated by potential benefits such as sensor miniaturization, multiplexing opportunities and higher sensitivities, refractometric nanoplasmonic biosensing has profiled itself in a short time span as an interesting alternative to conventional Surface Plasmon Resonance (SPR) biosensors. This latter conventional sensing concept has been subjected during the last decades to strong commercialization, thereby strongly leaning on well-developed thin-film surface chemistry protocols. Not surprisingly, the examples found in literature based on this sensing concept are generally characterized by extensive analytical studies of relevant clinical and diagnostic problems. In contrast, the more novel Localized Surface Plasmon Resonance (LSPR) alternative finds itself in a much earlier, and especially, more fundamental stage of development. Driven by new fabrication methodologies to create nanostructured substrates, published work typically focuses on the novelty of the presented material, its optical properties and its use – generally limited to a proof-of-concept – as a label-free biosensing scheme. Given the different stages of development both SPR and LSPR sensors find themselves in, it becomes apparent that providing a comparative analysis of both concepts is not a trivial task. Nevertheless, in this review we make an effort to provide an overview that illustrates the progress booked in both fields during the last five years. First, we discuss the most relevant advances in SPR biosensing, including interesting analytical applications, together with different strategies that assure improvements in performance, throughput and/or integration. Subsequently, the remaining part of this work focuses on the use of nanoplasmonic sensors for real label-free biosensing applications. First, we discuss the motivation that serves as a driving force behind this research topic, together with a brief summary that comprises the main fabrication methodologies used in this field. Next, the sensing performance of LSPR sensors is examined by analyzing different parameters that can be invoked in order to quantitatively assess their overall sensing performance. Two aspects are highlighted that turn out to be especially important when trying to maximize their sensing performance, being (1) the targeted functionalization of the electromagnetic hotspots of the nanostructures, and (2) overcoming inherent negative influence that stem from the presence of a high refractive index substrate that supports the nanostructures. Next, although few in numbers, an overview is given of the most exhaustive and diagnostically relevant LSPR sensing assays that have been recently reported in literature, followed by examples that exploit inherent LSPR characteristics in order to create highly integrated and high-throughput optical biosensors. Finally, we discuss a series of considerations that, in our opinion, should be addressed in order to bring the realization of a stand-alone LSPR biosensor with competitive levels of sensitivity, robustness and integration (when compared to a conventional SPR sensor) much closer to reality.     

Anchoring dyes into multidimensional large-pore zeolites: a prospective use as chromogenic sensing materials

A versatile procedure for anchoring dyes into the pores of multidimensional zeolites by including organic dye precursors in the synthesis gel has been developed. To prove the concept, an aniline-functionalised zeolite Beta was obtained by reaction of triethylorthosilicate (TEOS), tetraethylammonium hydroxide, and N-methyl,N-(propyl-3-trimethoxysilyl)aniline (MPTMSA) in the presence of HF. Further extraction of the structure-directing agents resulted in a highly crystalline, white, functionalised zeolite Beta containing anchored aniline groups. Similar organic functionalised molecular sieves (OFMS) have been explored as novel catalysts, but, as far as we know, OFMS have never been used as precursors for dye-immobilisation or to design new solid-based host systems for selective molecular sensing processes as is reported here. In a second step the solids containing dyes were prepared by reaction of the hybrid material with the appropriate reactives to obtain tricyanovinylbenzene, triphenylpyrylium, azoic, and squaraine derivatives. All these reactions are straightforward and involve electrophilic aromatic substitution or diazotisation reactions at the electron-rich aniline ring. The final dye-functionalised solid materials were isolated by simple filtration and washing procedures and have been characterised by a number of techniques. In all cases the Beta structure of the solid remains unaltered. Among the large number of areas where dye-containing zeolites might be of importance, we were interested in testing their unconventional use as heterosupramolecular hosts in chromogenic protocols. To check their potential use as chemosensors, microporous solids with anchored triphenylpyrilium and squaraine dyes were selected and used as sensors for the chromogenic discrimination of amines. It was found that the response of both solids to amines was basically governed by the three-dimensional (3D) solid architecture that tuned the intrinsic unselective reactivity of the pyrylium dye. By using new solid-state supramolecular chemistry protocols we believe that these, and similar future dye-zeolite hosts, might be promising new sensor materials allowing the visible discrimination of selected target guests by size and/or polarity within families or closely related molecules.     

(E)-3-(meso-Octamethylcalix[4]pyrrol-2-yl)propenal: a versatile precursor for calix[4]pyrrole-based chromogenic anion sensors

The synthesis of (E)-3-(meso-octamethylcalix[4]pyrrol-2-yl)propenal and its use in Knoevenagel reactions are described. The resulting compounds display sharp changes in color in the presence of fluoride, acetate, and dihydrogen phosphate anions.     

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.     

Synthesis of Cross‐Linked Polymeric Micelle pH Nanosensors: An Investigation of Design Flexibility

The design flexibility that polymeric micelles offer in the fabrication of optical nanosensors for ratiometric pH measurements is investigated. pH nanosensors based on polymeric micelles are synthesized either by a mixed‐micellization approach or by a postmicelle modification strategy. In the mixed‐micellization approach, self‐assembly of functionalized unimers followed by shell cross‐linking by copper‐catalyzed azide‐alkyne cycloaddition (CuAAC) results in stabilized cRGD‐functionalized micelle pH nanosensors. In the postmicelle modification strategy, simultaneous cross‐linking and fluorophore conjugation at the micelle shell using CuAAC results in a stabilized micelle pH nanosensor. Compared to the postmicelle modification strategy, the mixed‐micellization approach increases the control of the overall composition of the nanosensors. Both approaches provide stable nanosensors with similar pK_a profiles and thereby nanosensors with similar pH sensitivity.

     

Recent trends of biosensors in Japan

Remarkable progress has been made in the development of bioelectrochemical sensors, and various kinds of biosensors have been developed and applied in analytical chemistry. Many of these enzyme, microbial, and immunosensors have been recently developed in Japan and in this paper, recent trends in Japanese biosensor development are described.     

PEDOT films: multifunctional membranes for electrochemical ion sensing

Properties of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied from the point of view of direct use as ion-sensing membranes in potentiometric or amperometric sensors. Stable and reproducible potentiometric characteristics were obtained for PEDOT doped by poly(4-styrenesulfonate) ions, PEDOT(PSS) (cationic characteristics), and PEDOT doped by hexacyanoferrate(II) anions, PEDOT(HCF) (anionic characteristics). As shown by voltammetric and EDAX results, the anion exchange properties of the latter polymer result from gradual replacement of HCF ions by Cl^– anions from solution. The zero-current potentiometric detection limit of PEDOT(PSS), equal to 3×10^–6 M, can be shifted to 7×10^–7 M by polarization using a cathodic current density of 3×10^–7 A cm^–2. PEDOT films doped by Cl^– or PSS^– ions can be used as membranes for sensing anions or cations, respectively, under pulse amperometric conditions, within the range from 10^–4 to 1 M, comparable with that accessible by zero-current potentiometry. Dissolved oxygen (redox interferent of low charge transfer rate) exerts a minor influence on the slope of the potentiometric and amperometric characteristics of PEDOT films. Although the presence of redox reactants characterized by a high rate of charge transfer [Fe(CN)₆ ^3–/4–] results in the disappearance of the potential dependence on KCl concentration, this disadvantageous effect is much less significant under pulse amperometric conditions.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

Novel quinoxalinophenanthrophenazine-based molecules as sensors for anions: synthesis and binding investigations

The design and synthesis of two novel quinoxalinophenanthrophenazine-based anion sensors are reported. Binding studies of these sensors with an array of mono- and polyatomic anions using UV-vis, fluorescence, and NMR titrations have shown 1:1 and 1:2 sensor-to-anion ratios. Binding constants were calculated for anions, which exhibited high affinity for the sensors, including acetate, benzoate, cyanide, and fluoride ions.     

Analytical characteristics and sensitivity mechanisms of electrolyte-insulator-semiconductor system-based chemical sensors—a critical review

Recent trends in research and development of electrolyte-insulator-semiconductor (EIS) field-effect chemical sensors (ion-selective field-effect transistors, light-addressable potentiometric sensors, capacitive EIS-sensors) with inorganic gate insulators (oxide, nitride and chalcogenide films) are reviewed. Physical properties of EIS systems and basic mechanisms of their chemical sensitivity are examined. Analytical characteristics and sensing mechanisms of EIS pH sensors with oxide and nitride films, as well as metal ions sensors with chalcogenide films, are critically discussed. Prospects of future research on EIS field-effect biosensors are briefly outlined.     

A mini-review on MXenes as versatile substrate for advanced sensors

MXenes have emerged as versatile 2D materials that are already gaining paramount attention in the areas of energy,catalyst,electromagnetic shielding,and sensors.The unique surface chemistry,graphene-like mo rphology,high hydrophilicity,metal-like conductivity with redox capability identifies MXenes,as an ideal material for surface-related applications.This short review summarizes the most recent reports that discuss the potential application of MXenes and their hybrids as a transducer material for advanced sensors.Based on the nature of transducing signals,the discussion is categorized into three sections,which include electrochemical(bio) sensors,gas sensors,and finally,electro-chemiluminescence fluorescent sensors.The review provides a concise summary of all the analytical merits obtained subsequent to the use of MXenes,followed by endeavors that have been made to accentuate the future perspective of MXenes in sensor devices.     

Semiconductor Gas Sensors: Dry Synthesis and Application

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet‐ and dry‐deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono‐ or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non‐equilibrium or sub‐stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor‐deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.     

Resorcinarene-Encapsulated Nanoparticles: Building Blocks for Self-Assembled Nanostructures

Calix[4]resorcinarene-derived surfactants are highly effectiveat stabilizing metal nanoparticles of different sizes, creating opportunities tofabricate well-defined nanostructures with size-tunable materials properties. Theresorcinarenes have a critical role in the dispersion of nanoparticles under varioussolvent conditions and in the robustness of the protective surfactant layer.Magnetic cobalt particles stabilized by resorcinarenes self-assemble intonanostructured ``bracelets' in toluene. Resorcinarene surfactants can also promote theself-organization of gold nanoparticles as large as 170 nm into two-dimensional arrays. Thesenanostructured films possess novel optoelectronic properties such as surface-enhancedRaman scattering (SERS), and are expected to have useful applications for chemical sensing.