Ionic Liquid-Assisted Thermal Evaporation of Bimetallic Ag-Au Nanoparticle Films as a Highly Reproducible SERS Substrate for Sensitive Nanoplastic Detection in Complex Environments.

Nanoplastic particles are emerging as an important class of environmental pollutants in the atmosphere that have adverse effects on our ecosystems and human health. While many methods have been developed to quantitatively detect nanoplastics; however, sensitive detection at low concentrations in a complex environment remains elusive. Herein, we demonstrate a greener method to fabricate a surface-enhanced Raman spectroscopy (SERS) substrate consisting of self-assembled plasmonic Ag-Au bimetallic nanoparticle (NP) films for quantitative SERS detection of nanoplastics in complex media. The self-assembly of Ag-Au bimetallic NPs was achieved through thermal evaporation onto a vapor-phase compatible ionic liquid based on deep eutectic solvent over the growth substrate. The finite-difference time-domain simulation revealed that the localized field enhancement is strong in the gaps, which generate uniform SERS "hotspots" in the obtained substrate. Benefiting from highly accessible SERS "hotspots" at the gaps, the SERS substrate exhibits excellent sensitivity for detecting crystal violet with a limit of detection (LOD) as low as 10-14 M and excellent reproducibility (RSD of 5.8%). The SERS substrate is capable of detecting PET nanoplastics with LOD as low as 1 µg/mL and about 100 µg/mL in real samples such as tap water, lake water, diluted milk, and wine. Moreover, we also validated the feasibility of the designed SERS substrate for the practical detection of PET nanoplastics collected from commercial drinking water bottles, and it showed great potential applications for sensitive detection in actual environments.

Ni-Doped ZrO2 nanoparticles decorated MW-CNT nanocomposite for the highly sensitive electrochemical detection of 5-amino salicylic acid.

In this work, Ni-doped ZrO2 nanoparticles (NPs) were used to decorate multi-walled carbon nanotubes (MWCNTs) to obtain a Ni-ZrO2/MWCNT nanocomposite, which acted as an efficient electrode material for the highly sensitive electrochemical detection of the anti-inflammatory drug 5-amino salicylic acid (5-ASA). The Ni-ZrO2 NPs were obtained through a facile co-precipitation method, and the subsequent support of these Ni-ZrO2 NPs onto MWCNTs was accomplished via an ultrasonication technique. Supporting Ni-ZrO2 NPs on MWCNTs not only results in excellent catalytic properties, but it also substantially enhances the surface area, electrical conductivity, and electron transfer process. The electrochemical activity of the synthesized Ni-ZrO2/MWCNT nanocomposite was systematically investigated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The constructed Ni-ZrO2/MWCNT-modified glassy carbon (GC) electrode manifests superior electrocatalytic oxidation activity toward 5-ASA, with a lower peak potential compared with Ni-ZrO2-NP- and MWCNT-modified GC electrodes. Importantly, the proposed biosensor exhibited excellent sensitivity during the detection of 5-ASA with a wide linear concentration range (0.001-500 µM) and a low detection limit of 0.0029 µM. Moreover, the biosensor demonstrated excellent repeatability, reproducibility, stability, and high specificity toward 5-ASA detection in the presence of different interfering species. Furthermore, the biosensor showed satisfactory recovery rates in complex biological samples, such as human blood serum, human urine, and 5-ASA tablet samples.

Re-evaluating the role of phosphinic acid (DINHOP) adsorption at the photoanode surface in the performance of dye-sensitized solar cells.

Dineohexyl phosphinic acid (DINHOP) is a popular amphiphilic molecular insulator considered as the most efficient co-adsorbent (co-grafter) for the improvement of the photovoltaic performance of TiO2 based hybrid solar cells. Although the effect of its incorporation on the improvement of cell performance has been well demonstrated, the mechanisms through which it affects the photovoltaic and electrodynamic parameters of the cells are not yet clear. Here we re-examine the mechanism through which the DINHOP co-adsorbent affects the photovoltaic and electrodynamic parameters of dye-sensitized solar cells. Although DINHOP is widely believed to inhibit (passivate) recombination across the TiO2/electrolyte interface, we demonstrate that this is sublte, noticeable only for a very high concentration (e.g. 750 µM) of DINHOP, co-sensitized with a dye. For the most frequently used DINHOP concentrations (e.g. 75 µM and 375 µM), an observed increase of the diffusion coefficient and recombination rate could be directly associated with a decrease of total intra-gap states in TiO2. For a DINHOP concentration as low as 75 µM, the conduction band edge of TiO2 moves upward due to the combined effect of charge accumulation and a decrease in the total number of intra-gap states leading to an effective enhancement of the DCCS VOC, where the decrease in total intra-gap states does not contribute positively. The decrease of total intra-gap states enhances both the transport and recombination rates of charge carriers by the same fraction due to a transport-limited recombination process. On the other hand, adsorption of DINHOP molecules at higher concentrations such as 375 µM and 750 µM additionally modifies the distribution of intra-gap states, affecting the nonlinear recombination parameter of charge carriers at the anode-electrolyte interface, leading to an overall enhancement of the DSSC VOC. In all cases, incorporation of DINHOP results in an overall improvement of the solar cell efficiency (∼14% compared with the reference one), with a maximum for a concentration of 375 µM, where no inhibition of recombination was observed. Interestingly, for this DINHOP concentration, we estimate that 1 DINHOP molecule per every 12 molecules of dye occupies the intra-gap states of the TiO2 surface. The results presented in this work elucidate the physical phenomena involved in the interaction of co-adsorbents, pre-treatments or additives with the electrolyte at the surface of the TiO2 photoanode of dye-sensitized solar cells and can be easily adapted to study other electrochemical systems.

Oxygen-enriched carbon quantum dots from coffee waste: Extremely active organic photocatalyst for sustainable solar-to-H2O2 conversion.

Solar-driven artificial photosynthesis offers a promising avenue for hydrogen peroxide (H2O2) generation, an efficient and economical replacement for current methods. The efficiency and selectivity hurdles of the two-electron oxygen reduction reaction (ORR) in solar-to- H2O2 conversion are substantial barriers to large scale production. In this manuscript, a simple biomass-assisted synthesis was performed to produce oxygen-enriched carbon quantum dots (OE-CQDs) from spent coffee waste, acting as an efficient photocatalyst for solar-powered H2O2 production. OE-CQDs can stabilize and store light-generated electrons effectively, boosting charge separation and enhancing photocatalytic performance with longevity. The maximal photocatalytic H2O2 production was achieved viz the utilization of OE-CQDs with generation rate of 356.86 µmol g-1 h-1 by retaining 80% activity without any external sacrificial donors. The outstanding performance of synthesized OE-CQDs under light exposure at wavelength (λ) of 280 nm has been ensured by the quantum yield value of 9.4% upon H2O2 generation. The combinatorial benefits of OE-CQDs with their authentic crystalline structure and oxygen enrichment, is expected to be enhancing the ORR activity through accelerating charge transfer, and optimizing oxygen diffusion. Consequently, our eco-friendly method holds considerable promise for creating highly efficient, metal-free photocatalysts for artificial H2O2 production.

Deep Eutectic Solvent-Assisted Synthesis of Au Nanostars Supported on Graphene Oxide as an Efficient Substrate for SERS-Based Molecular Sensing.

The development of hybrid nanostructures of graphene oxide (GO) and metal nanoparticles (NPs) is of paramount interest for highly flexible surface-enhanced Raman scattering (SERS) substrate-based molecular sensing. In this work, we report a simple and eco-friendly synthesis strategy for the synthesis of a three-dimensional (3D) GO/gold nanostar (3D GO/Au NS) hybrid nanocomposite using deep eutectic solvent (DES) for SERS-based molecular sensing. The 3D GO/Au NS hybrid nanocomposite was obtained by a two-step synthetic process. In the first step, the GO nanosheets of thickness ∼1.25 nm were homogeneously dispersed in choline chloride/urea (molar ratio of 1:2)-derived DES, followed by functionalization of -NH groups using 3-aminopropyltriethoxysilane. Afterward, the presynthesized Au NSs of size ranging between 150-200 nm were then covalently attached on the -NH-functionalized GO nanosheets mediated by DES at 60 °C to obtain 3D GO/Au NS hybrid nanocomposites. Importantly, the SERS substrate fabricated using the 3D GO/Au NS hybrid nanocomposite exhibits highly enhanced SERS activity with an enhancement factor of 1.7 × 105 and high sensitivity for the detection of crystal violet with a concentration up to 10-11 M. The green synthetic approach presented here can be expected to be promising for the large-scale fabrication of GO-metal NP-based hybrid nanostructures for their potential applications in SERS-based sensing.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors.

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters. Over the last decade, different nanomaterials have been exploited to design highly efficient biosensors for the detection of analyte biomolecules. The discovery of graphene has spectacularly accelerated research on fabricating low-cost electrode materials because of its unique physical properties, including high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency. Graphene and its oxygenated derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), are becoming an important class of nanomaterials in the field of biosensors. The presence of oxygenated functional groups makes GO nanosheets strongly hydrophilic, facilitating chemical functionalization. Graphene, GO and rGO nanosheets can be easily combined with various types of inorganic nanoparticles, including metals, metal oxides, semiconducting nanoparticles, quantum dots, organic polymers and biomolecules, to create a diverse range of graphene-based nanocomposites with enhanced sensitivity for biosensor applications. This review summarizes the advances in two-dimensional (2D) and three-dimensional (3D) graphene-based nanocomposites as emerging electrochemical and fluorescent biosensing platforms for the detection of a wide range of biomolecules with enhanced sensitivity, selectivity and a low limit of detection. The biofunctionalization and nanocomposite formation processes of graphene-based materials and their unique properties, surface functionalization, enzyme immobilization strategies, covalent immobilization, physical adsorption, biointeractions and direct electron transfer (DET) processes are discussed in connection with the design and fabrication of biosensors. The enzymatic and nonenzymatic reactions on graphene-based nanocomposite surfaces for glucose- and cholesterol-related electrochemical biosensors are analyzed. This review covers a very broad range of graphene-based electrochemical and fluorescent biosensors for the detection of glucose, cholesterol, hydrogen peroxide (H2O2), nucleic acids (DNA/RNA), genes, enzymes, cofactors nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP), dopamine (DA), ascorbic acid (AA), uric acid (UA), cancer biomarkers, pathogenic microorganisms, food toxins, toxic heavy metal ions, mycotoxins, and pesticides. The sensitivity and selectivity of graphene-based electrochemical and fluorescent biosensors are also examined with respect to interfering analytes present in biological systems. Finally, the future outlook for the development of graphene based biosensing technology is outlined.

Seed-Mediated Growth of Ag@Au Nanodisks with Improved Chemical Stability and Surface-Enhanced Raman Scattering.

Bimetallic Ag@Au nanoparticles (NPs) have received significant research interest because of their unique optical properties and molecular sensing ability through surface-enhanced Raman scattering (SERS). However, the synthesis of Ag@Au core-shell plasmonic nanostructures with precisely controlled size and shape remained a great challenge. Here, we report a simple approach for the synthesis of bimetallic Ag@Au nanodisks of about 13.5 nm thickness and different diameters through a seed-mediated growth process. The synthesis involves the conformal deposition of Au atoms at the corner sites of Ag nanoplate (AgNPL) seeds coupled with site-selective oxidative etching of AgNPL edges to generate Ag@Au nanodisks. The resultant Ag@Au nanodisks manifest significantly improved chemical stability and tunable localized surface plasmon resonance from the visible to the near-infrared spectral range. Moreover, in comparison to AgNPLs, the Ag@Au nanodisks showed greatly enhanced SERS performance with an enhancement factor up to 0.47 × 105, which is nearly 3-fold higher than that of the original AgNPLs (0.18 × 105). Furthermore, the Ag@Au nanodisks show a high sensitivity for detecting probe molecules such as crystal violet of concentration as low as 10-9 M and excellent reproducibility, with the SERS intensity fluctuation less than 12.5%. The synthesis route adapted for the controlled fabrication of Ag@Au nanodisks can be a potential platform for maneuvering other bimetallic plasmonic nanostructures useful for plasmonics and sensing applications.

Chitosan-Covered Pd@Pt Core-Shell Nanocubes for Direct Electron Transfer in Electrochemical Enzymatic Glucose Biosensor.

Development of biosensors with high sensitivity, high spatial resolution, and low cost has received significant attention for their applications in medical diagnosis, diabetes management, and environment-monitoring. However, achieving a direct electrical contact between redox enzymes and electrode surfaces and enhancing the operational stability still remain as challenges. Inorganic metal nanocrystals (NCs) with precisely controlled shape and surface structure engineered with an appropriate organic coating can help overcome the challenges associated with their stability and aggregation for practical biosensor applications. Herein, we describe a facile, room-temperature, seed-mediated solution-phase route to synthesize monodisperse Pd@Pt core-shell nanocubes with subnanometer-thick platinum (Pt) shells. The enzyme electrode consisting of Pd@Pt core-shell NCs was first covered with a chitosan (CS) polymer and then glucose oxidase (GOx) immobilized by a covalent linkage to the CS. This polymer permits covalent immobilization through active amino (-NH) side groups to improve the stability and preserve the biocatalytic functions while the Pd@Pt NCs facilitate enhanced direct electron transfer (DET) in the biosensor. The resultant biosensor promotes DET and exhibits excellent performance for the detection of glucose, with a sensitivity of 6.82 µA cm-2 mM-1 and a wide linear range of 1-6 mM. Our results demonstrate that sensitive electrochemical glucose detection based on Pd@Pt core-shell NCs provides remarkable opportunities for designing low-cost and sensitive biosensors.