Using Nanomaterials as Excellent Immobilisation Layer for Biosensor Design

The endless development in nanotechnology has introduced new vitality in device fabrication including biosensor design for biomedical applications. With outstanding features like suitable biocompatibility, good electrical and thermal conductivity, wide surface area and catalytic activity, nanomaterials have been considered excellent and promising immobilisation candidates for the development of high-impact biosensors after they emerged. Owing to these reasons, the present review deals with the efficient use of nanomaterials as immobilisation candidates for biosensor fabrication. These include the implementation of carbon nanomaterials-graphene and its derivatives, carbon nanotubes, carbon nanoparticles, carbon nanodots-and MXenes, likewise their synergistic impact when merged with metal oxide nanomaterials. Furthermore, we also discuss the origin of the synthesis of some nanomaterials, the challenges associated with the use of those nanomaterials and the chemistry behind their incorporation with other materials for biosensor design. The last section covers the prospects for the development and application of the highlighted nanomaterials.

Interference-free photoelectrochemical immunoassays using carboxymethylated dextran-coated and gold-modified TiO2 nanotube arrays

An interference-free photoelectrochemical (PEC) immunoassay was developed for cardiac troponin I (cTnI) detection. Covalent linkage of cTnI antibody to carboxymethylated (CM-) dextran pre-immobilized onto a gold nanoparticles (AuNPs)-modified TiO₂ nanotube array (NTA) affords five consecutive analyte captures with surface regenerations in between. Changes in the photocurrents at this photoanode before and after cTnI captures can be well fitted with the Langmuir isotherm from 0.220 pM to 2.20 nM cTnI. Owing to the inherently high sensitivity of the PEC detection, the detection limit (2.20 pg/mL) is lower than the range attainable with the enzyme-linked immunosorbent assay (ELISA) (6.00-40.0 pg/mL). Furthermore, CM-dextran prevents species in complex biological matrices from nonspecifically adsorbing onto the sensor surface, a feature not attainable with uncoated semiconductor electrodes or those coated with non-hydrogel-based chemical modifiers. The excellent anti-fouling property of dextran hydrogel allowed us to validate the accuracy of our regenerable sensors through a comparison of PEC immunoassays of patient sera to those of ELISA.

The electrochemical detection of prostate specific antigen on glassy carbon electrode modified with combinations of graphene quantum dots, cobalt phthalocyanine and an aptamer

Herein, a novel aptasensor is developed for the electrochemical detection of prostate specific antigen (PSA) on electrode surfaces modified using various combinations of a Cobalt phthalocyanine (CoPc), an aptamer and graphene quantum dots (GQDs). Electrochemical impedance spectroscopy (EIS) as well as differential pulse voltammetry (DPV) are employed for the detection of PSA. In both analytical techniques, linear calibration curves were observed at a concentration range of 1.2-2.0 pM. The glassy carbon electrode where CoPc and GQDs are placed on the electrode when non-covalently linked followed by addition of the aptamer (GQDs-CoPc(ππ)-aptamer (sequential)) showed the best performance with a limit of detection (LoD) as low as 0.66 pM when using DPV. The detection limits were much lower than the dangerous levels reported for PSA in males tested for prostate cancer. This electrode showed selectivity for PSA in the presence of bovine serum albumin, glucose and L-cysteine. The aptasensor showed good stability, reproducibility and repeatability, deeming it a promising early detection device for prostate cancer.

Sodium Alginate Micelle-Encapsulating Zinc Phthalocyanine Dye-Sensitized Photoelectrochemical Biosensor with CdS as the Photoelectric Material for Hg2+ Detection

A simple and selective photoelectrochemical (PEC) biosensor was constructed for Hg²⁺ detection based on zinc phthalocyanine (ZnPc) dye-sensitized CdS using alginate not only as a carrier but also as a binder. First, CdS as a photoactive material was in situ modified on the electrode surface using a rapid and simple electrodeposition to obtain an initial photocurrent signal. Second, ZnPc was loaded in the amphiphilic alginate micelle and then was coated onto the CdS film surface via alginate as the binder. The photocurrent was subsequently enhanced due to the favorable dye sensitization effect of ZnPc to CdS. Finally, the thymine-rich probe DNA was immobilized on the modified ITO surface via coupling reaction between the carbonyl groups of the amphiphilic polymer and the amino groups of the probe DNA. In the presence of Hg²⁺, the thymine-Hg²⁺-thymine (T-Hg²⁺-T) structure was formed due to the specific bond of Hg²⁺ with thymine, resulting in the decrease of photocurrent due to the increase of steric hindrance on the modified electrode surface. The proposed PEC biosensor for Hg²⁺ detection possessed a wide linear range from 10 pM to 1.0 μM with a detection limit of 5.7 pM. This biosensor provides a promising platform for detecting other biomolecules of interest.

Oligonucleotide-modulated photocurrent enhancement of a tetracationic porphyrin for label-free homogeneous photoelectrochemical biosensing

This work reports the first demonstration of an oligonucleotide-modulated label-free homogeneous photoelectrochemical (PEC) biosensing platform based on the adsorption of tetracationic porphyrin (denoted as TMPyP here) onto 1-naphthalenesulfonate anion (NS^-)-grafted indium tin oxide electrode (denoted as TMPyP-NS^--ITO), which generates a stable and rapid photocurrent response. We found that when NS^--ITO electrode was subjected to single-stranded oligonucleotide (ssON) before TMPyP adsorption, a remarkable enhancement of photocurrent intensity was observed from the resulted TMPyP-ssON-NS^--ITO electrode with high specificity towards oligonucleotide. A series of investigations were carried out to understand the mechanism of this oligonucleotide-modulated photocurrent enhancement phenomenon. Moreover, the studies of this robust photocurrent enhancement mechanism was successfully extended to develop a signal-on homogeneous PEC biosensing platform for, as a proof-of-concept, label-free M.SssI methyltransferase activity analysis through a judiciously and compatibly engineered signal transduction strategy consisted of hairpin-shaped oligonucleotide probe, restriction endonuclease HpaII, and Exonuclease I. The rationally designed homogeneous PEC biosensor exhibit sensitive PEC response toward M.SssI methyltransferase with a low detection limit of 3.5 mU/mL and a wide linear range from 0.01 to 120 U/mL. Additionally, we show that our homogeneous PEC biosensing platform can be also utilized to screen methyltransferase inhibitors. Therefore, this work will provide a distinctive paradigm for versatile homogeneous PEC biosensing platform that can be used as potential powerful tool toward innovative label-free bioanalytical purposes.

Conjugated Polymer-Based Photoelectrochemical Cytosensor with Turn-On Enable Signal for Sensitive Cell Detection

In this work, a new photoelectrochemical (PEC) cytosensor was constructed by using cationic polyfluorene derivative, poly(9,9-bis(6'-(N,N,N,-trimethylammonium)hexyl)fluorene-co-alt-1,4-phenylene)bromide (PFP) as the photoelectric-responsive material for sensitive cell detection. Positive-charged PFP with high photoelectric conversion efficiency can generate robust photocurrent under light illumination. In the PEC cytosensor, 3-phosphonopropionic acid was linked to the indium tin oxide electrode, followed by modification with antiepithelial-cell-adhesion-molecule (EpCAM) antibody via amide condensation reaction. Thus, target SKBR-3 cells with overexpressed EpCAM antigen could be captured onto the electrode via the specific antibody-antigen interactions. Upon adding cationic PFP, a favorable electrostatic interaction between cationic PFP and negatively charged cell membrane led to a turn-on detection signal for target SKBR-3 cells. This new cytosensor not only exhibits good sensitivity because of the good photoelectric performance of conjugated polymers, but also offers decent selectivity to target cells by taking advantage of the specific antibody-antigen recognition.

Enhanced Photoelectrochemical Activity by Autologous Cd/CdO/CdS Heterojunction Photoanodes with High Conductivity and Separation Efficiency

The development for hydrogen from solar energy has attracted great attention due to the global demand for clean, environmentally friendly energy. Herein, autologous Cd/CdO/CdS heterojunctions were prepared in a carefully controlled process with metallic Cd as the inner layer and CdO as the interlayer. Further research revealed that the transportation and separation of photogenerated pairs were enhanced due to low resistance of the Cd inner layer and the type II CdO/CdS heterojunction. As a result, the optimized Cd/CdO/CdS heterojunction photoanode showed outstanding and long-term photoelectrochemical activity for water splitting, with a current density of 3.52 mA cm^-2 , or a benchmark specific hydrogen production rate of 1.65 μmol cm^-2  min^-1 at -0.3 V versus Ag/AgCl, by using the environmental pollutants of sulfide and sulfite as sacrificial agents.

Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples

Photoelectrochemical (PEC) bioanalysis is a newly developed methodology that provides an exquisite route for innovative biomolecular detection. Quantum dots (QDs) are semiconductor nanocrystals with unique photophysical properties that have attracted tremendous attentions among the analytical community. QDs-based PEC bioanalysis comprises an important research hotspot in the field of PEC bioanalysis due to its combined advantages and potentials. Currently, it has ignited increasing interests as demonstrated by increased research papers. This review aims to cover the most recent advances in this field. With the discussion of recent examples of QDs-PEC bioanalysis from the literatures, special emphasis will be placed on work reporting on fundamental advances in the signaling strategies of QDs-based PEC bioanalysis from 2013 to now. Future prospects in this field are also discussed.

Photoelectrochemical DNA Biosensor Based on Dual-Signal Amplification Strategy Integrating Inorganic-Organic Nanocomposites Sensitization with λ-Exonuclease-Assisted Target Recycling

Sensitive and accurate analysis of DNA is crucial to better understanding of DNA functions and early diagnosis of fatal disease. Herein, an enhanced photoelectrochemical (PEC) DNA biosensor was proposed based on dual-signal amplification via coupling inorganic-organic nanocomposites sensitization with λ-exonuclease (λ-Exo)-assisted target recycling. The short DNA sequence about chronic myelogenous leukemia (CML, type b3a2) was selected as target DNA (tDNA). ZnO nanoplates were deposited with CdS nanocrystals to form ZnO/CdS hetero-nanostructure, and it was used as PEC substrate for immobilizing hairpin DNA (hDNA). CdTe quantum dots (QDs) covalently linked with meso-tetra(4-carboxyphenyl)porphine (TCPP) to form CdTe/TCPP inorganic-organic nanocomposites, which were utilized as sensitization agents labeling at the terminal of probe DNA (pDNA). When the hDNA-modified sensing electrode was incubated with tDNA and λ-Exo, hDNA hybridized with tDNA, and meanwhile it could be recognized and cleaved by λ-Exo, resulting in the release of tDNA. The rest of nonhybridized hDNA would continuously hybridize with the released tDNA, cleave by λ-Exo, and set free the tDNA again. After λ-Exo-assisted tDNA recycling, more amounts of short DNA (sDNA) fragments coming from digestion of hDNA produced on the electrode and hybridized with CdTe/TCPP-labeled pDNA (pDNA-CdTe/TCPP conjugates). In this case, the sensitization of CdTe/TCPP inorganic-organic nanocomposites occurred, which evidently extend the absorption range and strengthened the absorption intensity of light energy, and accordingly the photocurrent signal significantly promoted. Through introducing the dual-signal amplification tactics, the developed PEC assay allowed a low calculated detection limit of 25.6 aM with a wide detection scope from 0.1 fM to 5 pM for sensitive and selective determination of tDNA.

Recent advances in the use of quantum dots for photoelectrochemical bioanalysis

Photoelectrochemical (PEC) bioanalysis is a newly developed technique for innovative biomolecular detection. Quantum dots (QDs) with unique photophysical properties are key components in realization of various exquisite PEC bioanalyses. Particularly, significant progress has been made in the QD-based PEC bioanalysis. In this work, we briefly summarize the most recent and important developments in the use of traditional and newly emerging QDs for novel PEC bioanalytical applications. The future prospects in this dynamic field are also highlighted.

Enhanced light-driven catalytic performance of cytochrome P450 confined in macroporous silica

A light-driven approach combined with a macroporous reactor for the enzymatic biocatalytic reaction has been developed by confining the enzyme/photosensitizer nanohybrids in a macroporous material, which exhibits high bio-conversion efficiency due to the fast diffusion and collision between the enzyme/photosensitizer nanohybrid and the substrate in the reactor.

Photoelectrochemical bioanalysis: the state of the art

This review provides a panoramic snapshot of the state of the art in the dynamically developing field of photoelectrochemical bioanalysis.

Ultrasensitive photoelectrochemical immunoassay for matrix metalloproteinase-2 detection based on CdS:Mn/CdTe cosensitized TiO2 nanotubes and signal amplification of SiO2@Ab2 conjugates

An ultrasensitive photoelectrochemical sandwich immunoassay was developed to detect matrix metalloproteinase-2 (MMP-2, antigen, Ag) based on CdS:Mn/CdTe cosensitized TiO2 nanotubes (TiO2-NTs) and signal amplification of SiO2@Ab2 conjugates. Specifically, the TiO2-NTs electrode was first deposited with CdS:Mn by successive ionic layer adsorption and reaction technique and then further coated with CdTe quantum dots (QDs) via the layer-by-layer method, forming TiO2-NTs/CdS:Mn/CdTe cosensitized structure, which was employed as a matrix to immobilize capture MMP-2 antibodies (Ab1); whereas, SiO2 nanoparticles were coated with signal MMP-2 antibodies (Ab2) to form SiO2@Ab2 conjugates, which were used as signal amplification elements via the specific antibody-antigen immunoreaction between Ag and Ab2. The ultrahigh sensitivity of this immunoassay derived from the two major reasons as below. First, the TiO2-NTs/CdS:Mn/CdTe cosensitized structure could adequately absorb the light energy, dramatically promote electron transfer, and effectively inhibit the electron-hole recombination, resulting in significantly enhanced photocurrent intensity of the sensing electrode. However, in the presence of target Ag, the immobilized SiO2@Ab2 conjugates could evidently increase the steric hindrance of the sensing electrode and effectively depress the electron transfer, leading to obviously decreased photocurrent intensity. Accordingly, the well-designed photoelectrochemical immunoassay exhibited a low detection limit of 3.6 fg/mL and a wide linear range from 10 fg/mL to 500 pg/mL for target Ag detection. Meanwhile, it also presented good reproducibility, specificity, and stability and might open a new promising platform for the detection of other important biomarkers.

Construction of photoelectrochemical thrombin aptasensor via assembling multilayer of graphene-CdS nanocomposites

A photoelectrochemical (PEC) aptasensor for highly sensitive and specific detection of thrombin was developed by using graphene–CdS nanocomposites multilayer as photoactive species and electroactive mediator hexaammineruthenium(III) chloride (Ru(NH(3))(6)(3+)) as signal enhancer. Graphene–CdS nanocomposites (G–CdS) were synthesized by one-pot reduction of oxide graphene and CdCl2 with thioacetamide. The photoactive multilayer was prepared by alternative assembly of the negatively charged 3-mercaptopropionic acid modified graphene–CdS nanocomposites (MPA-G–CdS) and the positively charged polyethylenimine (PEI) on ITO electrode. This layer-by-layer assembly method enhanced the stability and homogeneity of the photocurrent readout of G–CdS. Thrombin aptamer was covalently bound to the multilayer by using glutaraldehyde as cross-linking. Electroactive mediator (Ru(NH(3))(6)(3+)) could interact with the DNA phosphate backbone and thus facilitated the electron transfer between G–CdS multilayer and electrode and enhanced the photocurrent. Hybridizing of a long complementary DNA with thrombin aptamer could increase the adsorption amount of (Ru(NH(3))(6)(3+)), which in turn boosted the signal readout. In the presence of target thrombin, the affinity interaction between thrombin and its aptamer resulted in the long complementary DNA releasing from the G–CdS multilayer and decreasing of photocurrent signal. On the basis of G–CdS multilayer as the photoactive species, (Ru (NH(3))(6)(3+)) as an electroactive mediator, and aptamer as a recognition module, a high sensitive PEC aptasensor for thrombin detection was proposed. The thrombin aptasensor displayed a linear range from 2.0 pM to 600.0 pM and a detection limit of 1.0 pM. The present strategy provided a promising ideology for the future development of PEC biosensor.

Photoelectrochemical biosensor using enzyme-catalyzed in situ propagation of CdS quantum dots on graphene oxide

An innovative photoelectrochemical (PEC) biosensor platform was designed based on the in situ generation of CdS quantum dots (QDs) on graphene oxide (GO) using an enzymatic reaction. Horseradish peroxidase catalyzed the reduction of sodium thiosulfate with hydrogen peroxide to generate H2S, which reacted with Cd(2+) to form CdS QDs. CdS QDs could be photoexcited to generate an elevated photocurrent as a readout signal. This strategy offered a "green" alternative to inconvenient presynthesis procedures for the fabrication of semiconducting nanoparticles. The nanomaterials and assembly procedures were characterized by microscopy and spectroscopy techniques. Combined with immune recognition and on the basis of the PEC activity of CdS QDs on GO, the strategy was successfully applied to a PEC assay to detect carcinoembryonic antigen and displayed a wide linear range from 2.5 ng mL(-1) to 50 μg mL(-1) and a detection limit of 0.72 ng mL(-1) at a signal-to-noise ratio of 3. The PEC biosensor showed satisfactory performance for clinical sample detection and was convenient for determining high concentrations of solute without dilution. This effort offers a new opportunity for the development of numerous rapid and convenient analytical techniques using the PEC method that may be applied in the design and preparation of various solar-energy-driven applications.

Rapid and specific sensing of gallic acid with a photoelectrochemical platform based on polyaniline-reduced graphene oxide-TiO2

A novel photoelectrochemical sensor has been designed with polyaniline-reduced graphene oxide-titanium dioxide, which was further applied to sense gallic acid and exhibited extraordinary rapid response, high sensitivity and excellent anti-inference. Meanwhile, the mechanism has been elaborately explored.

In situ generated AgBr-enhanced ZnO nanorod-based photoelectrochemical aptasensing via layer-by-layer assembly

A robust aptasensor for Ag(+) was proposed for the first time using an enhanced ZnO nanorod-based photoelectrochemistry by in situ generated AgBr via layer-by-layer assembly. This work opens up new avenues for application of one-dimensional ZnO nanorod arrays in photoelectrochemical sensing. Additionally, the strategy of employing in situ generated narrow-bandgap semiconductors paves a new way for photoelectrochemical sensing.

Photoelectrochemical lab-on-paper device equipped with a porous Au-paper electrode and fluidic delay-switch for sensitive detection of DNA hybridization

The sequence-specific detection of DNA hybridization has attracted considerable interest in numerous fields. Although traditional DNA biosensors have been widely explored due to their high sensitivity, it is still challenging to develop a low-cost, portable, disposable, fast, and easy-to-use DNA detection method for public use at home or in the field. To address these challenges, herein, we report a novel microfluidic photoelectrochemical (PEC) paper-based analytical platform, integrated with an internal chemiluminescent light source, a novel paper supercapacitor (PS) amplifier, and a terminal digital multi-meter (DMM) detector, for sensitive DNA detection using a graphene-modified porous Au-paper electrode as the working electrode to obtain enhanced PEC responses. The quantification mechanism of this strategy is based on the charging of this PS, which was constructed on a paper-based analytical platform through a simple "drawing and soaking" method, by the generated photocurrent. After a fixed period, the PS was automatically shorted under the control of a novel built-in fluidic delay-switch to output an instantaneously amplified current, which could be sensitively detected by the DMM. At optimal conditions, this paper-based analytical platform can detect DNA at concentrations at femtomolar level. This approach also shows excellent specificity toward single nucleotide mismatches.

Ultrasensitive photoelectrochemical immunoassay of indole-3-acetic acid based on the MPA modified CdS/RGO nanocomposites decorated ITO electrode

A novel ultrasensitive photoelectrochemical immunosensor was fabricated based on 3-mercaptopropionic acid stabilized CdS/reduced graphene oxide (MPA-CdS/RGO) nanocomposites for indole-3-acetic acid (IAA) detection. The MPA-CdS/RGO nanocomposites were synthesized by in situ solvothermal growth of triangulated pyramidal CdS nanoparticles on the RGO sheet. 3-Mercaptopropionic acid (MPA) was employed as the modifier and bridge to immobilize the antibody. The nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and UV/vis spectra. The results showed that the MPA-CdS/RGO nanocomposites revealed enhanced photocurrent response due to excellent electron transport properties of RGO and the improved assembly of CdS nanoparticles onto RGO sheet with the introduction of MPA. Based on the dependence of the photocurrent decline on the concentration of IAA, the proposed photoelectrochemical immunosensor for IAA depicted a linear range from 0.1 to 1000 ng/mL with a lower detection limit (0.05 ng/mL). The high sensitivity, reproducibility and specificity of the method permitted the method suitable to be used in real samples.