Click Chemistry-Mediated Nanosensors for Biochemical Assays

Convenient and sensitive colorimetric detection of melamine in dairy products based on Cu(ii)-H2O2-3,3′,5,5′-tetramethylbenzidine system

The illegal adulteration of melamine in dairy products for false protein content increase is a strong hazard to human health. Herein, a simple and sensitive colorimetric method was developed for the quantification of melamine in dairy products based on a Cu²⁺-hydrogen peroxide (H₂O₂)-3,3′,5,5′-tetramethylbenzidine (TMB) system. In this strategy, Cu²⁺ exhibits peroxidase-like activity and can catalyze the oxidation of TMB to oxidized TMB (oxTMB) in the presence of H₂O₂ with a blue colour change of the solution. However, the presence of melamine quickly interacts with H₂O₂ leading to the consumption of H₂O₂ and thus strongly hinders the oxidation of TMB. Under the optimal conditions, the absorbance change of oxTMB has a linear response to the concentration of melamine from 1 to 100 μM with a detection limit of 0.5 μM for melamine. The proposed method has many merits including more simplicity, good selectivity, and more cost-effectiveness without using any nanomaterials. The method was further successfully applied to detect melamine in dairy products including milk and infant formula powder.

Convenient and sensitive colorimetric detection of melamine in dairy products based on a Cu(ii)-H₂O₂-3,3′,5,5′-tetramethylbenzidine system was reported.

Variably doped nanostructured gallium nitride surfaces can serve as biointerfaces for neurotypic PC12 cells and alter their behavior

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization. The semiconductor analysis included atomic force microscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy. The semiconductor surfaces were then evaluated as biointerfaces, and the in vitro cell behavior was quantified based on cell viability, reactive oxygen species production, as well as time dependent intracellular Ca concentration, [Ca²⁺]_i, a known cell-signaling molecule. In this work, we show that persistent photoconductivity (PPC) can be used to alter the surface properties prior to chemical functionalization, the concentration of dopants can have some effect on cellular behavior, and that chemical functionalization changes the surface potential before and after exposure to UV light. Finally, we describe some competing mechanisms of PPC-induced [Ca²⁺]_i changes, and how researchers looking to control cell behavior non-invasively can consider PPC as a useful control knob.

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

Cucurbituril and Azide Cofunctionalized Graphene Oxide for Ultrasensitive Electro-Click Biosensing

To achieve high selectivity and sensitivity simultaneously in an electrochemical biosensing platform, cucurbituril and azide cofunctionalized graphene oxide, a new functional nanomaterial that acts as a go-between to connect the recognition element with amplified signal architecture, is developed in this work. The cucurbituril and azide cofunctionalized graphene oxide features a high specific surface area with abundant levels of the two types of functional groups. Specifically, it emerges as a powerful tool to link recognition elements with simplicity, high yield, rapidity, and highly selective reactivity through azide-alkynyl click chemistry. Moreover, it possesses many host molecules to interact with guest molecules (also signal molecules)-grafted branched ethylene imine polymer, through which the detection sensitivity can be greatly improved. Together with electro-click technology, a highly controllable, selective, and sensitive biosensing platform can be easily created. For VEGF₁₆₅ protein detection, the electro-click assay has high selectivity and sensitivity; a dynamic detection range from 10 fg mL^-1 to 1 ng mL^-1 with a detection limit of 8 fg mL^-1 was achieved. The electro-click biosensing strategy based on cucurbituril and azide cofunctionalized graphene oxide would have great promise for other target analytes with a broad range of applications.

Plasmon-Based Colorimetric Nanosensors for Ultrasensitive Molecular Diagnostics

Colorimetric detection of target analytes with high specificity and sensitivity is of fundamental importance to clinical and personalized point-of-care diagnostics. Because of their extraordinary optical properties, plasmonic nanomaterials have been introduced into colorimetric sensing systems, which provide significantly improved sensitivity in various biosensing applications. Here we review the recent progress on these plasmonic nanoparticles-based colorimetric nanosensors for ultrasensitive molecular diagnostics. According to their different colorimetric signal generation mechanisms, these plasmonic nanosensors are classified into two categories: (1) interparticle distance-dependent colorimetric assay based on target-induced forming cross-linking assembly/aggregate of plasmonic nanoparticles; and (2) size/morphology-dependent colorimetric assay by target-controlled growth/etching of the plasmonic nanoparticles. The sensing fundamentals and cutting-edge applications will be provided for each of them, particularly focusing on signal generation and/or amplification mechanisms that realize ultrasensitive molecular detection. Finally, we also discuss the challenge and give our future perspective in this emerging field.

Therapeutic targeting in nanomedicine: the future lies in recombinant antibodies

The unique chemical and functional properties of nanoparticles can be harnessed for the delivery of large quantities of various therapeutic biomolecules. Active targeting of nanoparticles by conjugating ligands that bind to target cells strongly facilitates accumulation, internalization into target cells and longer retention at the target site, with consequent enhanced therapeutic effects. Recombinant antibodies with high selectivity and availability for a vast range of targets will dominate the future. In this review, we systematically outline the tremendous progress in the conjugation of antibodies to nanoparticles and the clear advantages that recombinant antibodies offer in the therapeutic targeting of nanoparticles. The demonstrated flexibility of recombinant antibody coupling to nanoparticles highlights the bright future of this technology for modern therapeutic nanomedicine.

Two-Way Gold Nanoparticle Label-Free Sensing of Specific Sequence and Small Molecule Targets Using Switchable Concatemers

A two-way colorimetric biosensor based on unmodified gold nanoparticles (GNPs) and a switchable double-stranded DNA (dsDNA) concatemer have been demonstrated. Two hairpin probes (H1 and H2) were first designed that provided the fuels to assemble the dsDNA concatemers via hybridization chain reaction (HCR). A functional hairpin (FH) was rationally designed to recognize the target sequences. All the hairpins contained a single-stranded DNA (ssDNA) loop and sticky end to prevent GNPs from salt-induced aggregation. In the presence of target sequence, the capture probe blocked in the FH recognizes the target to form a duplex DNA, which causes the release of the initiator probe by FH conformational change. This process then starts the alternate-opening of H1 and H2 through HCR, and dsDNA concatemers grow from the target sequence. As a result, unmodified GNPs undergo salt-induced aggregation because the formed dsDNA concatemers are stiffer and provide less stabilization. A light purple-to-blue color variation was observed in the bulk solution, termed the light-off sensing way. Furthermore, H1 ingeniously inserted an aptamer sequence to generate dsDNA concatemers with multiple small molecule binding sites. In the presence of small molecule targets, concatemers can be disassembled into mixtures with ssDNA sticky ends. A blue-to-purple reverse color variation was observed due to the regeneration of the ssDNA, termed the light-on way. The two-way biosensor can detect both nucleic acids and small molecule targets with one sensing device. This switchable sensing element is label-free, enzyme-free, and sophisticated-instrumentation-free. The detection limits of both targets were below nanomolar.

Bioorthogonal Reaction-Mediated ELISA Using Peroxide Test Strip as Signal Readout for Point-of-Care Testing

This work demonstrates a highly sensitive peroxide test strip (PTS)-based enzyme-linked immunosorbent assay (ELISA) for both qualitative and quantitative detection of drugs of abuse (morphine) and disease biomarkers (interleukin-6 and HIV-1 capsid antigen p24). This color-based PTS is a commercially available product with advantages of low cost, easy operation, and portability, and it is an ideal signal readout strategy in ELISA to simplify the immunoassay procedures and enable point-of-care testing (POCT). In addition, we introduce the bioorthogonal reaction that can effectively amplify the signal by controlling the cycles of bioorthogonal reaction to achieve the desirable sensitivity depending on different analytes. The limit of detection is 0.2 ng/mL for morphine, 3.98 pg/mL for interleukin-6, and 11.6 pg/mL for detection of HIV-capsid antigen (p24). This PTS-ELISA applies to both the qualitative and quantitative detection of IL-6 and p24 in clinical serum samples with good accuracy, which provides a promising tool for the POCT in clinical diagnosis.

Progress in the Development of Nanotheranostic Systems

This thematic issue includes both review and research articles and is intended to provide an overview on the recent progress in the development of nanostructure-based therapeutic, diagnostic, and theranostic systems.