Yuan-Cheng Zhu

Quantum Dots: Electrochemiluminescent and Photoelectrochemical Bioanalysis

In this Feature, electrochemiluminescent (ECL) and photoelectrochemical (PEC) properties and mechanisms of semiconductor quantum dots (QDs) are reviewed, with emphasis on their specific fundamentals and concise comparison on their similarities and differences. With recent illustrative examples of bioanalytical applications, the main signaling strategies for QDs-based ECL and PEC bioanalysis are then highlighted. The future prospects in this field are also discussed.

DNA Labeling Generates a Unique Amplification Probe for Sensitive Photoelectrochemical Immunoassay of HIV-1 p24 Antigen

Photoelectrochemical (PEC) immunoassay is an attractive methodology as it allows for an elegant and sensitive protein assay. However, advanced PEC immunoassay remains challenging and the established amplifications rely almost exclusively on the labeling of various enzymes, which usually suffer the inferior stabilities. Here we report the development and validation of the DNA labeling that leads to a unique amplification probe for the sensitive PEC immunoassay of HIV-1 capsid protein, p24 antigen, an important biomarker of human immune deficiency virus (HIV). Following the sandwich immunobinding, the DNA tags could be released and the subsequent dipurinization of the oligonucleotide strands enables the easy oxidation of free nucleobases at a CdTe quantum dots (QDs) modified ITO transducer. Such DNA tags induced PEC amplification and readout permits the exquisite assay of HIV-1 p24 antigen with high sensitivity. As compared to the existing method of enzymatic labeling, the easy preparation and stability of these labels make them very suitable for PEC amplification. Another merit of this method is that it separates the immunobinding from the PEC transducer, which eliminates the commonly existing affection during the biorecognition processes. This work paves a new route for the PEC immunoassay of HIV-1 p24 antigen and provides a general format for the PEC biomolecular detection by means of the DNA labeling.

Alkaline Phosphatase Tagged Antibodies on Gold Nanoparticles/TiO2 Nanotubes Electrode: A Plasmonic Strategy for Label-Free and Amplified Photoelectrochemical Immunoassay

This work reports a plasmonic strategy capable of label-free yet amplified photoelectrochemical (PEC) immunoassay for the sensitive and specific detection of model protein p53, an important transcription factor that regulates the cell cycle and functions as a tumor suppressor. Specifically, on the basis of Au nanoparticles (NPs) deposited on hierarchically ordered TiO2 nanotubes (NTs), a protein G molecular membrane was used for immobilization of alkaline phosphatase (ALP) conjugated anti-p53 (ALP-a-p53). Due to the immunological recognition between the receptor and target, the plasmonic charge separation from Au NPs to the conduction band of TiO2 NTs could be influenced greatly that originated from multiple factors. The degree of signal suppression is directly associated with the target concentration, so by monitoring the changes of the plasmonic photocurrent responding after the specific binding, a new plasmonic PEC immunoassay could be tailored for label-free and amplified detection. The operating principle of this study could be extended as a general protocol for numerous other targets of interest.

Photoelectrochemical Bioanalysis Platform of Gold Nanoparticles Equipped Perovskite Bi4NbO8Cl

We have developed sensitive photoelectrochemical (PEC) detection of cysteine using the gold nanoparticles (Au NPs) equipped perovskite Bi4NbO8Cl heterostructure. The Bi4NbO8Cl was prepared by a solid-state reaction, and the Au NPs/Bi4NbO8Cl electrode was made through the electrostatic layer-by-layer self-assembly technique. The Au NPs/Bi4NbO8Cl electrode provided much enhanced photocurrent with a great increase compared to the bare Bi4NbO8Cl electrode and allowed for the plasmon-enhanced PEC detection of cysteine with good performance. It demonstrated rapid response, high stability, wide linear detection range and certain selectivity, implying its great promise in its application. Therefore, the Au NPs/Bi4NbO8Cl heterostructure has provided a promising platform for the development of PEC bioanalysis. More generally, these findings offered an insight into the exploitation of perovskite materials for PEC bioanalytical purposes.

DNA sequence functionalized with heterogeneous core–satellite nanoassembly for novel energy-transfer-based photoelectrochemical bioanalysis

This work reports the use of compositionally heterogeneous asymmetric Ag@Au core-satellite nanoassembly functionalized with DNA sequence as unique signaling nanoprobes for the realization of new energy-transfer-based photoelectrochemical (PEC) immunoassay of prostate- specific antigen (PSA). Specifically, the Ag@Au asymmetric core-satellite nanoassemblies (Ag@Au ACS) were fabricated on a two-dimensional glass substrate by a modified controlled assembly technique, and then functionalized with DNA sequences containing PSA aptamers as signaling nanoprobes. Then, the sandwich complexing between the PSA, its antibodies, and the signaling nanoprobes was performed on a CdS QDs modified indium tin oxide (ITO) electrode. The single stranded DNA can server as a facile mediator that place the Ag@Au ACS in proximity of CdS QDs, stimulating the interparticle exciton-plasmon interactions between Ag@Au ACS and CdS QDs and thus quenching the excitonic states in the latter. Since the damping effect is closely related to the target concentration, a novel energy-transfer-based PEC bioanalysis could be achieved for the sensitive and specific PSA assay. The developed biosensor displayed a linear range from 1.0×10-11gmL-1 to 1.0×10-7gmL-1 and the detection limit was experimentally found to be of 0.3×10-13gmL-1. This strategy used the Ag@Au ACS-DNA signaling nanoprobes and overcame the deficiency of short operating distance of the energy transfer process for feasible PEC immunoassay. More significantly, it provided a way to couple the plasmonic properties of the Ag NPs and Au NPs in a single PEC bioanalytical system. We expected this work could inspire more interests and further investigations on the advanced engineering of the core-satellite or other judiciously designed nanostructures for new PEC bioanalytical uses with novel properties.

Semiconducting CuO Nanotubes: Synthesis, Characterization, and Bifunctional Photocathodic Enzymatic Bioanalysis

This work reports the synthesis, characterization, and application of bifunctional semiconducting CuO nanotubes (NTs) electrode for innovative synergized cathodic photoelectrochemical (PEC) enzymatic bioanalysis. Specifically, CuO NTs electrode was fabricated by surface oxidation of the copper foil in an alkaline aqueous solution with (NH4)2S2O8 and then annealed in air at 200 °C. After the subsequent coupling with the model enzyme of xanthine oxidase (XOD), the resulted photocathodic enzyme biosensor exhibited good analytical performance of rapid response, high stability, and good sensitivity. Especially, due to the unique catalytic property of CuO toward H2O2, a novel enzymatic cascade design between biological catalyst (XOD as natural enzyme) and biomimetic catalyst (CuO as the peroxidase mimetics) was constructed, and the dual-catalyst system with special synergy effect could achieve the cathodic PEC guanine bioanalysis with enhanced efficiency. In the determination, the cathodic photocurrent was found to be proportional to the guanine concentration, which was different from the commonly observed O2-dependent suppression of the photocurrent. In all, such a bifunctional CuO NTs-based PEC bioassay format has not been reported. The success of this work can offer great chances for further development and implementation of novel CuO-based PEC bioanalytical systems. More importantly, the strategy proposed here could contribute to the development of an original prototype for general PEC enzymatic bioanalysis.

Ferroelectric Perovskite Oxide@TiO2 Nanorod Heterostructures: Preparation, Characterization, and Application as a Platform for Photoelectrochemical Bioanalysis

This work reports the first synthesis and characterization of a ferroelectric perovskite oxide-based heterostructure as well as its application for photoelectrochemical (PEC) bioanalytical purposes. Specifically, exemplified by [KNbO3]1- x[BaNi1/2Nb1/2O3-δ] x (KBNNO), the ferroelectric perovskite oxides were prepared by solid-state synthesis, while the TiO2 nanorod (NR) arrays were obtained via a hydrothermal method. Using the technique of pulsed laser deposition (PLD), KBNNO were then deposited on TiO2 NRs to form KBNNO@TiO2 NR heterostructures. Various characterization techniques were applied to reveal compositional and structural information on the as-fabricated sample, and favorable alignment existed between the two components as displayed by the PEC test. In the detection of l-cysteine, the as-fabricated KBNNO@TiO2 NRs demonstrated good performance in terms of sensitivity and selectivity. This work revealed the potential of ferroelectric perovskite oxide and its heterostructures for innovative PEC bioanalytical applications, and we hope it will generate more interest in the development of various ferroelectrics-based heterostructures for advanced PEC bioanalysis.

3D Semiconducting Polymer/Graphene Networks: Toward Sensitive Photocathodic Enzymatic Bioanalysis

This work reports the development of three-dimensional (3D) semiconducting polymer/graphene (SP/G) networks toward sensitive photocathodic enzymatic bioanalysis. Specifically, the porous 3D graphene was first synthesized via the hydrothermal and freeze-dry processes and then mixed with semiconducting polymer to obtain the designed hierarchical structure with unique porosity and large surface area. Afterward, the as-prepared hybrid was immobilized onto the indium tin oxide (ITO) for further characterizations. Exemplified by sarcosine oxidase (SOx) as a model biocatalyst, an innovative 3D SP/G-based photocathodic bioanalysis capable of sensitive and specific sarcosine detection was achieved. The suppression of cathodic photocurrent was observed in the as-developed photocathodic enzymatic biosystem due to the competition of oxygen consumption between the enzyme-biocatalyst process and O2-dependent photocathodic electrode. This work not only presented a unique protocol for 3D SP/G-based photocathodic enzymatic bioanalysis but also provided a new horizon for the design, development, and utilization of numerous 3D platforms in the broad field of general photoelectrochemical (PEC) bioanalysis.