Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule Raman spectroscopy

Template-confined assembly of Ag nanocubes: An approach to fabricate SERS substrate with good performance

Surface-enhanced Raman scattering (SERS) is an important analytical technique. Its detection sensitivity and reproducibility depend on the density and distribution of SERS hotspots. Self-assembly is an efficient method to produce of SERS substrates due to its easy accessibility. However, the assembled defects can hardly be avoided on large area, which could lower the density and uniformity of the hotspots, leading to poor SERS performance. Herein, we report a method to reduce the defects by taking a patterned substrate as template to confine the assembly of Ag nanocubes. The template was prepared based on the combination of photo lithography and self-assembly. Confined by the template, the Ag nanocubes were assembled closely in each dots of the pattern. The limit of detection (LOD) is down to 3.42 × 10-17 M and the enhanced factor (EF) is up to 3.44 × 1010 on the prepared substrate for detecting rhodamine 6G (R6G). In addition, the relative standard deviation (RSD) of the different substrates is 8.75 %. The assembled Ag nanocubes exhibits high sensitivity and reproducibility as SERS substrate, which are contributed by the formation of high-density and uniform hotspots. The prepared substrate can be used for detecting trace amounts of melamine in milk with LOD of 2.06 × 10-7 M and RSD of 6.91 %, so the substrate is applicable for analyzing various analytes.

Strategies to improve hydrogen activation on gold catalysts

Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.

Holistic Prediction of AuNP Aggregation in Diverse Aqueous Suspensions Based on Machine Vision and Dark-Field Scattering Imaging

The localized surface-plasmon resonance of the AuNP in aqueous media is extremely sensitive to environmental changes. By measuring the signal of plasmon scattering light, the dark-field microscopic (DFM) imaging technique has been used to monitor the aggregation of AuNPs, which has attracted great attention because of its simplicity, low cost, high sensitivity, and universal applicability. However, it is still challenging to interpret DFM images of AuNP aggregation due to the heterogeneous characteristics of the isolated and discontinuous color distribution. Herein, we introduce machine vision algorithms for the training of DFM images of AuNPs in different saline aqueous media. A visual deep learning framework based on AlexNet is constructed for studying the aggregation patterns of AuNPs in aqueous suspensions, which allows for rapid and accurate identification of the aggregation extent of AuNPs, with a prediction accuracy higher than 0.96. With the aid of machine learning analysis, we further demonstrate the prediction ability of various aggregation phenomena induced by both cation species and the concentration of the external saline solution. Our results suggest the great potential of machine vision frameworks in the accurate recognition of subtle pattern changes in DFM images, which can help researchers build predictive analytics based on DFM imaging data.

A SERS aptasensor based on a flexible substrate for interference-free detection of carbendazim in apple

Non-destructive and interference-free monitoring of pesticide residue on the surface of fruits is still a challenge. Herein, a SERS aptasensor based on a flexible substrate was established for effective carbendazim (CBZ) detection on apple peel. In this sensor, electrospun PVDF/CQDs film served as a flexible supporting substrate. AuNS@Ag was liquid-liquid self-assembled on the PVDF/CQDs film to form a uniform and highly active SERS substrate. During the detection process, aptamers specifically capture the CBZ molecules, while nitrile-mediated Raman tag (MMBN) linked to AuNPs provided optical anti-interference signals. The results showed that the developed sensor had high sensitivity, selectivity, reproducibility, and stability for CBZ detection. Importantly, the flexibility of the SERS substrate helped the sensor realize non-invasive CBZ detection at a concentration as low as 1.20 ng/cm2 on apple peel, which is much lower than the maximum residue limits of CBZ in apples.

Theory and modeling of light-matter interactions in chemistry: current and future

Light-matter interaction not only plays an instrumental role in characterizing materials' properties via various spectroscopic techniques but also provides a general strategy to manipulate material properties via the design of novel nanostructures. This perspective summarizes recent theoretical advances in modeling light-matter interactions in chemistry, mainly focusing on plasmon and polariton chemistry. The former utilizes the highly localized photon, plasmonic hot electrons, and local heat to drive chemical reactions. In contrast, polariton chemistry modifies the potential energy curvatures of bare electronic systems, and hence their chemistry, via forming light-matter hybrid states, so-called polaritons. The perspective starts with the basic background of light-matter interactions, molecular quantum electrodynamics theory, and the challenges of modeling light-matter interactions in chemistry. Then, the recent advances in modeling plasmon and polariton chemistry are described, and future directions toward multiscale simulations of light-matter interaction-mediated chemistry are discussed.

Photocatalysis of Metallic Nanoparticles: Interband vs Intraband Induced Mechanisms

Photocatalysis induced by localized surface plasmon resonance of metallic nanoparticles has been studied for more than a decade, but photocatalysis originating from direct interband excitations is still under-explored. The spectral overlap and the coupling of these two optical regimes also complicate the determination of hot carriers' energy states and eventually hinder the accurate assignment of their catalytic role in studied reactions. In this Featured Article, after reviewing previous studies, we suggest classifying the photoexcitation via intra- and interband transitions where the physical states of hot carriers are well-defined. Intraband transitions are featured by creating hot electrons above the Fermi level and suitable for reductive catalytic pathways, whereas interband transitions are featured by generating hot d-band holes below the Fermi level and better for oxidative catalytic pathways. Since the contribution of intra- and interband transitions are different in the spectral regions of localized surface plasmon resonance and direct interband excitations, the wavelength dependence of the photocatalytic activities is very helpful in assigning which transitions and carriers contribute to the observed catalysis.

Au Nanoparticles Decorated CoP Nanowire Array: A Highly Sensitive, Anticorrosive, and Recyclable Surface-Enhanced Raman Scattering Substrate

Metal-semiconductor composites are promising candidates for surface-enhanced Raman scattering (SERS) substrates, but their inert basal plane, poor active sites, and limited stability hamper their commercial prospects. Herein, we report a three-dimensional CoP nanowire array decorated with Au nanoparticles on carbon cloth (Au@CoP/CC) as a self-supporting flexible SERS substrate. The Au nanoparticles spontaneously grew on the surface of the CoP nanowire array to form efficient SERS hot spots by a redox reaction with HAuCl₄ without any additional reducing agents. Such Au@CoP/CC substrate exhibited a limit of detection of 10^-11 M using rhodamine 6G as a model dye with outstanding corrosion resistance ability even under extreme acid and alkali conditions, which is better than many recently reported Au-based SERS substrates. Finite-difference time-domain simulation results demonstrated that Au@CoP/CC can provide a high density of regions with intense local electric field enhancement. Moreover, Au@CoP/CC can degrade target organic dyes for the self-cleaning and reproduction of SERS-active substrates under visible light irradiation. This work provides a novel means of using the plasmonic metal-transition metal phosphide composites for high-performance SERS sensing and photodegradation.

ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs). Benefiting from the synergistic effect of electromagnetic enhancement and charge transfer effect, the Ag NFs@ZnO NRs substrate exhibits a low detection limit (10^-13 M) for crystal violet molecules. This SERS substrate has good uniformity with a relative standard deviation of 7.463 %. Besides, owning to the photocatalytic property of ZnO NRs, the hybrid substrate can degrade probe molecules after SERS detection and realize recyclability. As a demonstration, we employed our SERS substrate for the trace detection of pesticide residues on apple surface and in river water. This study provides a new idea for improving the SERS performance of hybrid substrates.

Synthesis of mesoporous Ag/α-Fe2O3/TiO2 heterostructures with enhanced and accelerated photo/-catalytic reduction of 4-nitrophenol

4-Nitrophenol (4-NP) is reported to originate disadvantageous effects on the human body collected from industrial pollutants; therefore, the detoxification of 4-NP in aqueous contamination is strongly recommended. In this study, the heterojunction mesoporous α-Fe₂O₃/TiO₂ modulated with diverse Ag percentages has been constructed via a sol-gel route in the occurrence of a soft template P123. The formation of biphasic crystalline TiO₂ anatase and brookite phases has been successfully achieved with the average 10 nm particle sizes. The photo/-catalytic reduction of 4-NP has been performed utilizing NaBH₄ as a reducing agent with and without visible illumination. All Ag/Fe₂O₃/TiO₂ nanocomposites exhibited significantly higher photo/-catalytic reduction efficiency than pure Fe₂O₃, TiO₂ NPs, and Fe₂O₃/TiO₂ nanocomposite. 2.5% Ag/Fe₂O₃/TiO₂ nanocomposite was considered the highest and superior photocatalytic reduction efficiency, and it almost achieved 98% after 9 min. Interestingly, the photocatalytic reduction of 4-NP was accelerated 9 times higher than the catalytic reduction over 2.5% Ag/Fe₂O₃/TiO₂; its rate constant value was 709 and 706 times larger than pure TiO₂ and Fe₂O₃ NPs, respectively. The enhanced photocatalytic reduction ability of Ag/Fe₂O₃/TiO₂ nanocomposite might be referred to as significantly providing visible light absorption and a large surface area, and it can upgrade the effective separation and mobility of electron holes. The stability of the synthesized catalysts exhibited that the obtained catalysts can undergo a slight decrease in reduction efficiency after five successive cycles. This approach highlights a novel route for constructing ternary nanocomposite systems with high photo/-catalytic ability.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Metal cluster plasmons analyzed by energy-resolved photoemission

The optical response of size-selected metal clusters is studied by wavelength-dependent photoemission and energy-resolved photoelectron detection. Relative photodetachment cross sections giving information on the plasmon are determined for the example of closed-shell Ag₉₁^-. Notably, the peak energy of this anion (3.74 eV) is higher than the small particle limit in Mie theory of 3.5 eV. Different methods to extract cross sections from the spectra are applied. In particular, we compare the results obtained by integrating the full electron yields to analyses based on evaluating specified binding energy windows. The approach opens up new possibilities to conduct studies on Landau fragmentation as a result of multielectron excitations.

The Novel Role of Mitochondrial Citrate Synthase and Citrate in the Pathophysiology of Alzheimer's Disease

Citrate synthase is a key mitochondrial enzyme that utilizes acetyl-CoA and oxaloacetate to form citrate in the mitochondrial membrane, which participates in energy production in the TCA cycle and linked to the electron transport chain. Citrate transports through a citrate malate pump and synthesizes acetyl-CoA and acetylcholine (ACh) in neuronal cytoplasm. In a mature brain, acetyl-CoA is mainly utilized for ACh synthesis and is responsible for memory and cognition. Studies have shown low citrate synthase in different regions of brain in Alzheimer's disease (AD) patients, which reduces mitochondrial citrate, cellular bioenergetics, neurocytoplasmic citrate, acetyl-CoA, and ACh synthesis. Reduced citrate mediated low energy favors amyloid-β (Aβ) aggregation. Citrate inhibits Aβ25-35 and Aβ1-40 aggregation in vitro. Hence, citrate can be a better therapeutic option for AD by improving cellular energy and ACh synthesis, and inhibiting Aβ aggregation, which prevents tau hyperphosphorylation and glycogen synthase kinase-3 beta. Therefore, we need clinical studies if citrate reverses Aβ deposition by balancing mitochondrial energy pathway and neurocytoplasmic ACh production. Furthermore, in AD's silent phase pathophysiology, when neuronal cells are highly active, they shift ATP utilization from oxidative phosphorylation to glycolysis and prevent excessive generation of hydrogen peroxide and reactive oxygen species (oxidative stress) as neuroprotective action, which upregulates glucose transporter-3 (GLUT3) and pyruvate dehydrogenase kinase-3 (PDK3). PDK3 inhibits pyruvate dehydrogenase, which decreases mitochondrial-acetyl-CoA, citrate, and cellular bioenergetics, and decreases neurocytoplasmic citrate, acetyl-CoA, and ACh formation, thus initiating AD pathophysiology. Therefore, GLUT3 and PDK3 can be biomarkers for silent phase of AD.

Au Nanoparticles (NPs) Decorated Co Doped ZnO Semiconductor (Co400-ZnO/Au) Nanocomposites for Novel SERS Substrates

Au nanoparticles were decorated on the surface of Co-doped ZnO with a certain ratio of Co²⁺/Co³⁺ to obtain a novel semiconductor-metal composite. The optimal substrate, designated as Co₄₀₀-ZnO/Au, is beneficial to the promotion of separation efficiency of electron and hole in a semiconductor excited under visible laser exposure, which the enhances localized surface plasmon resonance (LSPR) of the Au nanoparticles. As an interesting finding, during Co doping, quantum dots of ZnO are generated, which strengthen the strong semiconductor metal interaction (SSSMI) effect. Eventually, the synergistic effect effectively advances the surface enhancement Raman scattering (SERS) performance of Co₄₀₀-ZnO/Au composite. The enhancement mechanism is addressed in-depth by morphologic characterization, UV-visible, X-ray diffraction, photoluminescence, X-ray photoelectron spectroscopy, density functional theory, and finite difference time domain (FDTD) simulations. By using Co₄₀₀-ZnO/Au, SERS detection of Rhodamine 6G presents a limit of detection (LOD) of 1 × 10^-9 M. As a real application, the Co₄₀₀-ZnO/Au-based SERS method is utilized to inspect tyramine in beer and the detectable concentration of 1 × 10^-8 M is achieved. In this work, the doping strategy is expected to realize a quantum effect, triggering a SSSMI effect for developing promising SERS substrates in future.

SERS monitoring of photoinduced-enhanced oxidative stress amplifier on Au@carbon dots for tumor catalytic therapy

Currently, artificial enzymes-based photodynamic therapy (PDT) is attractive due to its efficient capacity to change the immunosuppressive tumor microenvironment (TME). It is of great significance to study the therapeutic mechanism of novel artificial enzymes in TME through a monitoring strategy and improve the therapeutic effect. In this study, Au@carbon dots (Au@CDs) nanohybrids with a core-shell structure are synthesized, which not only exhibit tunable enzyme-mimicking activity under near-infrared (NIR) light, but also excellent surface-enhanced Raman scattering (SERS) properties. Therefore, Au@CDs show a good capability for monitoring NIR-photoinduced peroxidase-like catalytic processes via a SERS strategy in tumor. Moreover, the Au@CDs deplete glutathione with the cascade catalyzed reactions, thus elevating intratumor oxidative stress amplifying the reactive oxygen species damage based on the NIR-photoinduced enhanced peroxidase and glutathione oxidase-like activities, showing excellent and fast PDT therapeutic effect promoted by photothermal property in 3 min, finally leading to apoptosis in cancer cells. Through SERS monitoring, it is further found that after removing the NIR light source for 33 min, the reactive oxygen species (ROS) activity of the TME is counteracted and eliminated due to the presence of glutathione. This work presents a guidance to rationally design of artificial enzyme for ROS-involved therapeutic strategies and a new spectroscopic tool to evaluate the tumor catalytic therapy.

Plasmonically enhanced electrochemistry boosted by nonaqueous solvent

Plasmon excitation of metal electrodes is known to enhance important energy related electrochemical transformations in aqueous media. However, the low solubility of nonpolar gases and molecular reagents involved in many energy conversion reactions limits the number of products formed per unit time in aqueous media. In this Communication, we use linear sweep voltammetry to measure how electrochemical H2O reduction in a nonaqueous solvent, acetonitrile, is enhanced by excitation of a plasmonic electrode. Plasmonically excited electrochemically roughened Au electrodes are found to produce photopotentials as large as 175 mV, which can be harnessed to lower the applied electrical bias required to drive the formation of H2. As the solvent polarity increases, by an increase in the concentration of H2O, the measured photopotential rapidly drops off to ∼50 mV. We propose a mechanism by which an increase in the H2O concentration increasingly stabilizes the photocharged plasmonic electrode, lowering the photopotential available to assist in the electrochemical reaction. Our study demonstrates that solvent polarity is an essential experimental parameter to optimize plasmonic enhancement in electrochemistry.

Review on LSPR assisted photocatalysis: effects of physical fields and opportunities in multifield decoupling

Since nano scale local surface plasmon resonance (LSPR) can broaden the visible absorption region, enhance the local electromagnetic field and produce a thermal effect simultaneously, the appropriate utilization of the LSPR effect is a noteworthy research direction towards visible light driven photocatalysts with high efficiency and low cost. In this study, the influence mechanism of the optical, electric, magnetic, and thermal physical fields on the photocatalytic efficiency of the LSPR system is for the first time reviewed, based on which the research bottlenecks of this method including the accurate predesign and regulation of the photocatalyst, the interpretation of electron movement and energy transfer mechanism, are specifically analyzed. Due to the micro-nano localization of LSPR, auxiliary methods are needed to reflect the micro electromagnetic and temperature field distribution which are otherwise formidable to measure experimentally. Alternatively, numerical methods with decoupling calculations of nano-scale physical fields are necessary to develop. Therefore, the development potential of different numerical simulation methods including mainstream FDTD, FEM and DDA is subsequently expounded, providing opportunities in resolving the bottleneck issues associated with photocatalysis. It is worth mentioning that although many important advances have been achieved in the preparation and application of LSPR assisted photocatalysts, the convincing function mechanism of LSPR is still lacking due to its multifield synergistic enhancement effect.

Two-Dimensional TiVC Solid-Solution MXene as Surface-Enhanced Raman Scattering Substrate

Two-dimensional (2D) MXenes are attractive candidates as surface-enhanced Raman scattering (SERS) substrates because of their metallic conductivity and abundant surface terminations. Herein, we report the facile synthesis of bimetallic solid-solution TiVC (MXene) and its application in SERS. The few-layered MXene nanosheets with high crystallinity were successfully prepared using a one-step chemical etching method without ultrasonic and organic solvent intercalation steps. SERS activity of the as-prepared MXene was investigated by fabricating free-standing TiVC film as the substrate. A SERS enhancement factor of 10¹² and femtomolar-level detection limit were confirmed using rhodamine 6G as a model dye with 532 nm excitation. The fluorescent signal of the rhodamine 6G dye was effectively quenched, making the SERS spectrum clearly distinguishable. Furthermore, we demonstrate that the TiVC-analyte system with ultrahigh sensitivity is dominated by the chemical mechanism (CM) based on the experimental and simulation results. The abundant density of states near the Fermi level of the TiVC and the strong interaction between the TiVC and analyte promote the intermolecular charge transfer resonance in the TiVC-analyte complex, resulting in significant Raman enhancement. Additionally, several other probe molecules were used for SERS detection to further verify CM-based selectivity enhancement on the TiVC substrates. This work provides guidance for the facile synthesis of 2D MXene and its application in ultrasensitive SERS detection.

A Review: Potential Application and Outlook of Photothermal Therapy in Oral Cancer Treatment

As one of the most common malignant tumors, oral cancer threatens people's health worldwide. However, traditional therapies, including surgery, radiotherapy, and chemotherapy can't meet the requirement of cancer cure. Photothermal therapy (PTT) has attracted widespread attentions for its advantages of the noninvasive process, few side effects, and promising tumor ablation. Up to now, three types of photothermal agents (PTAs) have been widely employed in oral cancer therapies, which involve metallic materials, carbon-based materials, and organic materials. Previous research mainly introduced hybrid materials due to benefits from the synergistic effect of multiple functions. In this review, we present the advancement of each type PTAs for oral cancer treatment in recent years. In each part, we introduce the properties and synthesis of each PTA, summarize the current studies, and analyze their potential applications. Furthermore, we discuss the status quo and the deficiencies hindering the clinical application of PTT, based on which gives the perspective of its future developing directions.

Mechanistic insight into deep holes from interband transitions in Palladium nanoparticle photocatalysts

Utilizing hot electrons generated from localized surface plasmon resonance is of widespread interest in the photocatalysis of metallic nanoparticles. However, hot holes, especially generated from interband transitions, have not been fully explored for photocatalysis yet. In this study, a photocatalyzed Suzuki-Miyaura reaction using mesoporous Pd nanoparticle photocatalyst served as a model to study the role of hot holes. Quantum yields of the photocatalysts increase under shorter wavelength excitations and correlate to “deeper” energy of the holes from the Fermi level. This work suggests that deeper holes in the d-band catalyze the oxidative addition of aryl halide R-X onto Pd⁰ at the nanoparticles' surface to form R-Pd^II-X complex, thus accelerating the rate-determining step of the catalytic cycle. The hot electrons do not play a decisive role. In the future, catalytic mechanisms induced by deep holes should deserve as much attention as the well-known hot electron transfer mechanism.

•

Comparison of quantum yield across different wavelengths

•

Interband transitions from shorter wavelength excitation offering deeper holes

•

Deeper holes with stronger oxidizing power for higher quantum yield

Indirect to Direct Charge Transfer Transition in Plasmon-Enabled CO2 Photoreduction

Understanding hot carrier dynamics between plasmonic nanomaterials and its adsorbate is of great importance for plasmon-enhanced photoelectronic processes such as photocatalysis, optical sensing and spectroscopic analysis. However, it is often challenging to identify specific dominant mechanisms for a given process because of the complex pathways and ultrafast interactive dynamics of the photoelectrons. Here, using CO₂ reduction as an example, the underlying mechanisms of plasmon-driven catalysis at the single-molecule level using time-dependent density functional theory calculations is clearly probed. The CO₂ molecule adsorbed on two typical nanoclusters, Ag₂₀ and Ag₁₄₇ , is photoreduced by optically excited plasmon, accompanied by the excitation of asymmetric stretching and bending modes of CO₂ . A nonlinear relationship has been identified between laser intensity and reaction rate, demonstrating a synergic interplay and transition from indirect hot-electron transfer to direct charge transfer, enacted by strong localized surface plasmons. These findings offer new insights for CO₂ photoreduction and for the design of effective pathways toward highly efficient plasmon-mediated photocatalysis.

In situ mass analysis of surface reactions using surface-enhanced Raman spectroscopy covering a wide range of frequencies

Both the structural change and mass change of adsorbates in heterogeneous surface reactions were simultaneously measured in situ using frequency-extended SERS spectroscopy.

Effect of Photocharging on Catalysis of Metallic Nanoparticles

The photocharging effect is widely known across different research fields, has rarely been quantified as a background contribution in photocatalysis, and has often been overlooked in mechanistic interpretation of nanoparticle photocatalysts. To address these issues, this work presents a two-step experiment: charging colloidal Pd nanoparticles with light and hole scavengers and using the charged particles to catalyze the reduction of 4-nitrophenol by NaBH₄ under non-irradiation conditions. Experimental kinetics demonstrated a proportional correlation between accumulated electrons and catalytic improvement of Pd nanoparticles. This work reminds us that photocharged nanoparticles may still catalyze chemical reactions as a background phenomenon even when they are not undergoing photoexcitation.

Utilization of layered double hydroxides (LDHs) and their derivatives as photocatalysts for degradation of organic pollutants

Direct or indirect discharge of wastes containing organic pollutants have contributed to the environmental pollution globally. Decontamination of highly polluted natural resources such as water using an effective treatment is a great challenge for public health and environmental protection. Photodegradation of organic pollutants using efficient photocatalyst has attracted extensive interest due to their stability, effectiveness towards degradation efficiency, energy, and cost efficiency. Among various photocatalysts, layered double hydroxides (LDHs) and their derivatives have shown great potential towards photodegradation of organic pollutants. Herein, we review the mechanism, key factors, and performance of LDHs and their derivatives for the photodegradation of organic pollutants. LDH-based photocatalysts are classified into three different categories namely unmodified LDHs, modified LDHs, and calcined LDHs. Each LDH category is reviewed separately in terms of their photodegradation efficiency and kinetics of degradation. In addition, the effect of photocatalyst dose, pH, and initial concentration of pollutant as well as photocatalytic mechanisms are also summarized. Lastly, the stability and reusability of different photocatalysts are discussed. Challenges related to modeling the LDHs and its derivatives are addressed in order to improve their functional capacity.

Luminescence enhancement of lead halide perovskite light-emitting diodes with plasmonic metal nanostructures

Metal halide perovskites, as newly emerging light emitters, have been attracting considerable attention on luminescent materials and devices, due to their superior optoelectronic properties and potential practical applications. Recently, perovskite light-emitting diodes (PeLEDs) based on lead halide perovskites (LHPs) have been largely designed and intensively studied in laboratory platforms. However, to satisfy demand and promote their commercialization, it is crucial to improve the efficiency and stability of PeLEDs. Accordingly, the surface-plasmon (SP) effect provides a promising approach to enhance their luminescence, which is realized by incorporating plasmonic metal nanostructures (NSs) into PeLEDs. This review presents a comprehensive overview of the research status and prospect on LHP-based plasmonic PeLEDs together with the corresponding perovskite light-emission films (PeLEFs). Firstly, the recent development of the PeLEDs is briefly introduced. Secondly, the mechanisms and photophysics of the PeLEDs by SP manipulation are simply illustrated and analyzed. Then, the recent progress and achievements on the theoretical and experimental results of SP effect applications in the PeLEDs together with PeLEFs are presented in detail and systematically reviewed. Next, the current challenges and future directions of the PeLEDs are shown and discussed. Finally, a critical summary and outlook of the PeLEDs are summarized and proposed. Our results indicate that this new class of LHP-based plasmonic PeLEDs presents future research fields and demonstrates promising applications in lighting and displays, and further luminescence enhancement in exciton radiation processes and light extraction techniques are a hopeful route to obtain high-performance PeLEDs.

Heterogeneous UV-Switchable Au nanoparticles decorated tungstophosphoric acid/TiO2 for efficient photocatalytic degradation process

In the present study, gold nanoparticles were locally well-decorated on the surface of TiO₂ using the tungstophosphoric acid (HPW), as UV-switchable reducing intermediate linkers. The prepared Au NPs/HPW/TiO₂ nanostructure was characterized using FTIR, XRD, EDS, SEM and TEM, which confirmed the successful attachment of quasi-spherical Au NPs in the range of 20-30 nm on the surface of HPW modified TiO₂. Also, the FTIR results show that the Au NPs were binded to TiO₂ through the terminal the oxygen atoms HPW. The photocatalytic performance of prepared nanostructures was assessed in degradation of nitrobenzene. The nitrobenzene photodegradation kinetic study revealed that it well followed the Langmuir-Hinshelwood kinetic model with the apparent rate constant of 0.001 min^-1 using anatase TiO₂, 0.0004 min^-1 using HPW, 0.0014 using HPW/TiO₂, while it was obtained 0.0065 min^-1 using Au NPs@HPW/TiO₂ nanostructure. It shows that the photocatalytic rate of the prepared nanocomposites increased by 6.5- and 4.6-fold compared to photoactivity of anatase TiO₂ and HPW/TiO₂ respectively. Also, the photocatalytic mechanism of process was proposed. Moreover, the reusability study confirmed that its photocatalytic activity still remained high after three cycles.

Light-Off in Plasmon-Mediated Photocatalysis

In plasmon-mediated photocatalysis it is of critical importance to differentiate light-induced catalytic reaction rate enhancement channels, which include near-field effects, direct hot carrier injection, and photothermal catalyst heating. In particular, the discrimination of photothermal and hot electron channels is experimentally challenging, and their role is under keen debate. Here we demonstrate using the example of CO oxidation over nanofabricated neat Pd and Au50Pd50 alloy catalysts, how photothermal rate enhancement differs by up to 3 orders of magnitude for the same photon flux, and how this effect is controlled solely by the position of catalyst operation along the light-off curve measured in the dark. This highlights that small fluctuations in reactor temperature or temperature gradients across a sample may dramatically impact global and local photothermal rate enhancement, respectively, and thus control both the balance between different rate enhancement mechanisms and the way strategies to efficiently distinguish between them should be devised.

Flow and extraction of energy and charge carriers in hybrid plasmonic nanostructures

Strong interactions of electromagnetic fields with plasmonic nanomaterials have been exploited in various applications. These applications have centred on plasmon-enhanced scattering rates in nearby molecules or plasmon-induced heating. A question that has emerged recently is whether it is possible to use plasmonic nanostructures in a range of hot electron (hole) applications, including photocatalysis, photovoltaics and photodetection. These applications require coupling of a plasmonic component, which amplifies the interaction of light with the material, to an attached non-plasmonic component that extracts this energy in the form of electronic excitations to perform a function. In this Perspective, we discuss recent work in the emerging field of hybrid plasmonics. We focus on fundamental questions related to the nanoscopic flow of energy and excited charge carriers in these multicomponent materials. We also address critical misconceptions, challenges and opportunities that require more attention.

Recent Progress on Metal-Enhanced Photocatalysis: A Review on the Mechanism

Metal-enhanced photocatalysis has recently received increasing interest, mainly due to the ability of metal to directly or indirectly degrade pollutants. In this review, we briefly review the recent breakthroughs in metal-enhanced photocatalysis. We discussed the recent progress of surface plasmon resonance (SPR) effect and small size effect of metal nanoparticles on photocatalysis; in particular, we focus on elucidating the mechanism of energy transfer and hot electron injection/transfer effect of metal nanoparticles and clusters while as photocatalysts or as cophotocatalysts. Finally, we discuss the potential applications of metal-enhanced photocatalysis, and we also offer some perspectives for further investigations.

Process Window for Seeded Growth of Arrays of Quasi-Spherical Substrate-Supported Au Nanoparticles

The controlled growth of surface-supported metal nanoparticles (NPs) is essential to a broad range of applications. To this end, we explore the seeded growth of highly ordered arrays of substrate-supported Au NPs through a fully orthogonal design of experiment (DoE) scheme applied to a reaction system consisting of HAuCl₄, citrate, and hydrogen peroxide. Scanning electron microscopy in combination with digital image analysis (DIA) is used to quantitatively characterize the resultant NP populations in terms of both particle and array features. The effective optical properties of the NP arrays are additionally analyzed using spectroscopic ellipsometry (SE), allowing characteristics of the localized surface plasmon resonances (LSPRs) of the arrays to be quantified. We study the dependence of the DIA- and SE-extracted features on the different reagent concentrations through modeling using multiple linear regression with backward elimination of independent variables. A process window is identified for which uniform arrays of quasi-spherical Au NPs are grown over large surface areas. Aside from reagent concentrations the system is highly sensitive to the hydrodynamic conditions during the deposition. This issue is likely caused by an Au precursor mass-transport limitation of the reduction reaction and it is found that agitation of the growth medium is best avoided to ensure a macroscopically even deposition. Parasitic homogeneous nucleation can also be a challenge and was separately studied in a full DoE scheme with equivalent growth media but without substrates, using optical tracking of the solutions over time. Conditions yielding quasi-spherical surface-supported NPs are found to also be affiliated with strong tendencies for parasitic homogeneous nucleation and thereby loss of Au precursor, but addition of polyvinyl alcohol can possibly help alleviate this issue.

Wavelength-Dependent Optical Nonlinear Absorption of Au-Ag Nanoparticles

The nonlinear optical absorption properties of Au-Ag nanoparticles (NPs) were studied using an open-aperture Z-scan under a nanosecond pulsed laser with wavelengths of 450 nm, 510 nm, 550 nm, and 600 nm. The experimental results demonstrated that, when the laser intensity was 1.04 × 1013 W/m2, the Au-Ag NPs showed saturated absorption (SA). When the laser intensity was increased to 3.03 × 1013 W/m2, the switch from SA to reverse saturation absorption (RSA) occurred. The nonlinear absorption and its transformation were analyzed by using local surface plasmon resonance (LSPR) effect, bleaching of ground state plasmon, and free carrier absorption theory.

Divalent Ion-Induced Switch in DNA Cleavage of KpnI Endonuclease Probed through Surface-Enhanced Raman Spectroscopy

We demonstrate the remarkable ability of surface-enhanced Raman spectroscopy (SERS) to track the allosteric changes in restriction endonuclease KpnI (R.KpnI) caused by metal ions. R.KpnI binds and promiscuously cleaves DNA upon activation by Mg²⁺ ions. However, the divalent ion Ca²⁺ induces high fidelity cleavage, which can be overcome by higher concentrations of Mg²⁺ ions. In the absence of any 3D crystal structure, for the first time, we have elucidated the structural underpinnings of such a differential effect of divalent ions on the endonuclease activity. A combined SERS and molecular dynamics (MD) approach showed that Ca²⁺ ion activates an enzymatic switch in the active site, which is responsible for the high fidelity activity of the enzyme. Thus, SERS in combination with MD simulations provides a powerful tool for probing the link between the structure and activity of enzyme molecules that play vital roles in DNA transactions.

Near-Infrared-Driven Photocatalysts: Design, Construction, and Applications

Photocatalysts, which utilize solar energy to catalyze the oxidation or reduction half reactions, have attracted tremendous interest due to their great potential in addressing increasingly severe global energy and environmental issues. Solar energy utilization plays an important role in determining photocatalytic efficiencies. In the past few decades, many studies have been done to promote photocatalytic efficiencies via extending the absorption of solar energy into near-infrared (NIR) light. This Review comprehensively summarizes the recent progress in NIR-driven photocatalysts, including the strategies to harvest NIR photons and corresponding photocatalytic applications such as the degradation of organic pollutants, water disinfection, water splitting for H₂ and O₂ evolution, CO₂ reduction, etc. The application of NIR-active photocatalysts employed as electrocatalysts is also presented. The subject matter of this Review is designed to present the relationship between material structure and material optical properties as well as the advantage of material modification in photocatalytic reactions. It paves the way for future material design in solar energy-related fields and other energy conversion and storage fields.

Elucidating the Role of Dissolved Organic Matter and Sunlight in Mediating the Formation of Ag-Au Bimetallic Alloy Nanoparticles in the Aquatic Environment

Elucidating the interactions between metal ions and dissolved organic matter and deciphering mechanisms for their mineralization in the aquatic environment are central to understanding the speciation, transport, and toxicity of nanoparticles (NPs). Herein, we examine the interactions between Ag⁺ and Au³⁺ ions in mixed solutions (χ_Ag = 0.2, 0.5, and 0.8) in the presence of humic acids (HAs) under simulated sunlight; these conditions result in the formation of bimetallic Ag-Au NPs. A key distinction is that the obtained alloy NPs are compositionally and morphologically rather different from NPs obtained from thermally activated dark processes. Photoillumination triggers a distinctive plasmon-mediated process for HA-assisted reductive mineralization of ions to bimetallic alloy NPs which is not observed in its dark thermal reduction counterpart. The initial nucleation of bimetallic NPs is dominated by differences in the cohesive energies of Ag and Au crystal lattices, whereas the growth mechanisms are governed by the strongly preferred incorporation of Ag ions, which stems from their greater photoreactivity. The bimetallic NPs crystallize in shapes governed by the countervailing influence of minimizing free energy through the adoption of Wulff constructions and the energetic penalties associated with twin faults. As such, assessments of the stability and the potential toxic effects of bimetallic NPs arising from their possible existence in aquatic environments will depend sensitively on the origins of their formation.

Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis

Plasmonic nanomaterials coupled with catalytically active surfaces can provide unique opportunities for various catalysis applications, where surface plasmons produced upon proper light excitation can be adopted to drive and/or facilitate various chemical reactions. A brief introduction to the localized surface plasmon resonance and recent design and fabrication of highly efficient plasmonic nanostructures, including plasmonic metal nanostructures and metal/semiconductor heterostructures is given. Taking advantage of these plasmonic nanostructures, the following highlights summarize recent advances in plasmon-driven photochemical reactions (coupling reactions, O₂ dissociation and oxidation reactions, H₂ dissociation and hydrogenation reactions, N₂ fixation and NH₃ decomposition, and CO₂ reduction) and plasmon-enhanced electrocatalytic reactions (hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, alcohol oxidation reaction, and CO₂ reduction). Theoretical and experimental approaches for understanding the underlying mechanism of surface plasmon are discussed. A proper discussion and perspective of the remaining challenges and future opportunities for plasmonic nanomaterials and plasmon-related chemistry in the field of energy conversion and storage is given in conclusion.

Accurate SERS monitoring of the plasmon mediated UV/visible/NIR photocatalytic and photothermal catalytic process involving Ag@carbon dots

The excited carriers (electrons and holes) and heat energy that originate from plasmonic metal nanomaterials are crucial to the enhancement of the photocatalytic performance. In this study, an Ag@carbon dots (Ag@CDs) hybrid has been prepared with excellent Fenton-like photocatalytic and photothermal conversion properties for catalyzing H₂O₂ to generate hydroxyl radicals (˙OH) for the degradation of crystal violet (CV) dye under full solar spectrum irradiation based on a unique plasmon effect. We have obtained some intrinsic kinetics information, including the reaction rate and apparent activation energy on the surface of the Ag@CDs, through a surface-enhanced Raman scattering strategy to investigate the contributions made by photocatalytic and photothermal effects in the plasmon mediated reaction under irradiation from ultraviolet (UV)/visible/near-infrared (NIR) light. In the visible light region, the Ag@CDs + H₂O₂ system exhibits the fastest apparent reaction rate owing to the involvement of a large number of hot carriers, which are generated by the strongest plasmon effect, and the presence of the photothermal effect mediated by the plasmonic effect. As the wavelength of the illumination blue-shifts to the UV region, the plasmon effect is weakened, resulting in a decrease in the number of hot carriers. Furthermore, the hot carriers will not be further thermalized because of interband transitions. In addition, the catalytic performance of Ag@CDs in the NIR region is almost dominated by the photothermal effect. This work provides deep insights into understanding the plasmon-mediated photocatalytic mechanism of the Ag@CDs hybrid.

Selectivity control of organic chemical synthesis over plasmonic metal-based photocatalysts

The factors, issues, and design of plasmonic metal-based photocatalysts for selective photosynthesis of organic chemicals have been discussed.

Recent Progress on Metal-Enhanced Photocatalysis: A Review on the Mechanism

Metal-enhanced photocatalysis has recently received increasing interest, mainly due to the ability of metal to directly or indirectly degrade pollutants. In this review, we briefly review the recent breakthroughs in metal-enhanced photocatalysis. We discussed the recent progress of surface plasmon resonance (SPR) effect and small size effect of metal nanoparticles on photocatalysis; in particular, we focus on elucidating the mechanism of energy transfer and hot electron injection/transfer effect of metal nanoparticles and clusters while as photocatalysts or as cophotocatalysts. Finally, we discuss the potential applications of metal-enhanced photocatalysis, and we also offer some perspectives for further investigations.

Charge-Transfer Resonance and Electromagnetic Enhancement Synergistically Enabling MXenes with Excellent SERS Sensitivity for SARS-CoV-2 S Protein Detection

The outbreak of coronavirus disease 2019 has seriously threatened human health. Rapidly and sensitively detecting SARS-CoV-2 viruses can help control the spread of viruses. However, it is an arduous challenge to apply semiconductor-based substrates for virus SERS detection due to their poor sensitivity. Therefore, it is worthwhile to search novel semiconductor-based substrates with excellent SERS sensitivity. Herein we report, for the first time, Nb₂C and Ta₂C MXenes exhibit a remarkable SERS enhancement, which is synergistically enabled by the charge transfer resonance enhancement and electromagnetic enhancement. Their SERS sensitivity is optimized to 3.0 × 10⁶ and 1.4 × 10⁶ under the optimal resonance excitation wavelength of 532 nm. Additionally, remarkable SERS sensitivity endows Ta₂C MXenes with capability to sensitively detect and accurately identify the SARS-CoV-2 spike protein. Moreover, its detection limit is as low as 5 × 10^-9 M, which is beneficial to achieve real-time monitoring and early warning of novel coronavirus. This research not only provides helpful theoretical guidance for exploring other novel SERS-active semiconductor-based materials but also provides a potential candidate for the practical applications of SERS technology.

From SERS to TERS and Beyond: Molecules as Probes of Nanoscopic Optical Fields

A detailed understanding of the interaction between molecules and plasmonic nanostructures is important for several exciting developments in (bio)molecular sensing and imaging, catalysis, as well as energy conversion. While much of the focus has been on the nanostructures that generate enhanced and nano-confined optical fields, we herein highlight recent work from our groups that uses the molecular response in surface and tip enhanced Raman scattering (SERS and TERS, respectively) to investigate different aspects of the local fields. TERS provides access to ultra-confined volumes, and as a result can further explore and explain ensemble-averaged SERS measurements. Exciting and distinct molecular behaviors are observed in the quantum limit of plasmons, including molecular charging, chemical conversion, and optical rectification. Evidence of multipolar Raman scattering from molecules additionally provides insights into the inhomogeneous electric fields that drive SERS and TERS and their spatial and temporal gradients. The time scales of these processes show evidence of cooperative nanoscale phenomena that altogether contribute to SERS and TERS.

Recent Advances in Plasmon-Promoted Organic Transformations Using Silver-Based Catalysts

Plasmonics has emerged as a promising methodology to promote chemical reactions and has become a field of intense research effort. Ag nanoparticles (NPs) as plasmonic catalysts have been extensively studied because of their remarkable optical properties. This review analyzes the emergence and development of localized surface plasmon resonance (LSPR) in organic chemistry, mainly focusing on the discovery of novel reactions with new mechanisms on Ag NPs. Initially, the basics of LSPR and LSPR-promoted photocatalytic mechanisms are illustrated. Then, the recent advances in plasmonic nanosilver-mediated photocatalysis in organic transformations are highlighted with an emphasis on the related reaction mechanisms. Finally, a proper perspective on the remaining challenges and future directions in the field of LSPR-promoted organic transformations is proposed.

Theoretical and Experimental Studies of Ti3C2 MXene for Surface-Enhanced Raman Spectroscopy-Based Sensing

Recent advances in MXenes with high carrier mobility show great application prospects in the surface-enhanced Raman scattering (SERS) field. However, challenges remain regarding the improvement of the SERS sensitivity. Herein, an effective strategy considering charge-transfer resonance for semiconductor-based substrates is presented to optimize the SERS sensitivity with the guidance of the density functional theory calculation. The theoretical calculation predicted that the excellent SERS enhancement for methylene blue (MeB) on Ti₃C₂ MXene can be excited by both 633 and 785 nm lasers, and the Raman enhanced effect is mainly originated from the charge-transfer resonance enhancement. In this work, the Ti₃C₂ MXenes exhibit an excellent SERS sensitivity with an enhancement factor of 2.9 × 10⁶ and a low detection limit of 10^-7 M for MeB molecules. Furthermore, the SERS enhancement of Ti₃C₂ and Au-Ti₃C₂ substrates exhibit higher selectivity on different molecules, which contributes to the detection of target molecules in complex solution environments. This work can provide some theoretical and experimental basis for the research on SERS activity of other MXene materials.

Kinetically Manipulating the Nucleus Attachment to Create Atypical Defective Rh-Pt Alloyed Nanostructures as Active Electrocatalysts

Defective metal nanostructures have attracted great attention due to the striking catalytic behavior of the defect sites. Atypical metal nanocrystals generated from attached nuclei can accommodate abundant grain boundaries (GBs) and twin boundaries (TBs). However, the understanding of their growth-mechanism and precisely synthetic control over such defective nanocrystals are still scarce. Herein, using the Rh-Pt nanoalloy as a model system, we systematically demonstrate that a prudent control of the reaction kinetics can manipulate the metal nucleation and nucleus attachment to create atypical nanocrystals, including small isolated nanoparticles (NPs), defect-rich wavy nanowires (WNWs), and {100} facet-bounded spliced nanocubes (SNCs). In the ethanol oxidation electrocatalysis, the Rh₄₇ Pt₅₃ WNWs featured with abundant TBs and GBs show the greatest mass activity (0.655 A ⋅ mg^-1 _Pt , 2.9 times to the commercial Pt/C) and durability. Our work captures the core of reaction kinetics on regulating the nucleus attachment and enables the rational control over the nanocrystal morphology and defect.

Vis-Responsive Copper-Modified Titania for Decomposition of Organic Compounds and Microorganisms

Seven commercial titania (titanium(IV) oxide; TiO2) powders with different structural properties and crystalline compositions (anatase/rutile) were modified with copper by two variants of a photodeposition method, i.e., methanol dehydrogenation and water oxidation. The samples were characterized by diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Although zero-valent copper was deposited on the surface of titania, oxidized forms of copper, post-formed in ambient conditions, were also detected in dried samples. All samples could absorb visible light (vis), due to localized surface plasmon resonance (LSPR) of zero-valent copper and by other copper species, including Cu2O, CuO and CuxO (x:1-2). The photocatalytic activities of samples were investigated under both ultraviolet (UV) and visible light irradiation (>450 nm) for oxidative decomposition of acetic acid. It was found that titania modification with copper significantly enhanced the photocatalytic activity, especially for anatase samples. The prolonged irradiation (from 1 to 5 h) during samples’ preparation resulted in aggregation of copper deposits, thus being detrimental for vis activity. It is proposed that oxidized forms of copper are more active under vis irradiation than plasmonic one. Antimicrobial properties against bacteria (Escherichia coli) and fungi (Aspergillus niger) under vis irradiation and in the dark confirmed that Cu/TiO2 exhibits a high antibacterial effect, mainly due to the intrinsic activity of copper species.