Stable and tunable plasmon resonance of molybdenum oxide nanosheets from the ultraviolet to the near-infrared region for ultrasensitive surface-enhanced Raman analysis

Recent advances in semiconductor nanostructure-based surface-enhanced Raman scattering sensors

Surface-enhanced Raman scattering (SERS) has become a transformative analytical tool, attracting growing interest for its wide-ranging applications. The development of SERS-active materials is now a central research area, spurring innovation in various types of SERS substrates. While noble metal-based substrates remain extensively studied, semiconductor-based, non-metal substrates are garnering attention due to their unique advantages: excellent chemical stability, high carrier mobility, biocompatibility, and precise fabrication control. However, their generally weaker enhancement effects limit their utility, underscoring the need for strategies to boost their SERS activity. Understanding the complex enhancement mechanisms in semiconductor-based SERS substrates is critical for designing next-generation materials with metal-like enhancement factors (EFs). The interplay of charge transfer, localized surface plasmon resonance, and photonic effects makes the enhancement process inherently challenging to unravel. Therefore, the search for new materials with exciting optoelectronic properties, as well as more innovative solutions to increase their SERS sensitivity, continues to grow. In this review, we explore the latest advancements in semiconductor-based SERS substrates, dissecting the complex enhancement mechanisms and various modification strategies aimed at achieving metal-like high EFs. We present a comprehensive analysis of the methods used to improve the SERS performance of semiconductor substrates and conclude with potential future directions for advancing this dynamic field.

TiO2 nanorod arrays@PDA/Ag with biomimetic polydopamine as binary mediators for duplex SERS detection of illegal food dyes

Based on TiO2 nanorod arrays@PDA/Ag (TNRs@PDA/Ag), a better surface-enhanced Raman scattering (SERS) sensor with effective enrichment and enhancement was investigated for duplex SERS detection of illicit food dyes. Biomimetic PDA functions as binary mediators by utilizing the structural characteristics of polydopamine (PDA), which include the conjugated structure and abundant hydrophilic groups. One PDA functioned as an electron transfer mediator to enhance the efficiency of electron transfer, and the other as an enrichment mediator to effectively enrich rhodamine B (RhB) and crystal violet (CV) through hydrogen bonding, π-π stacking, and electrostatic interactions. Individual and duplex detection of illicit food dyes (RhB and CV) was performed using TNRs@PDA/Ag to estimate SERS applications. Their linear equations and limits of detection of 1 nM for RhB and 5 nM for CV were derived. Individual and duplex food colour detection was successfully accomplished even in genuine chili meal with good results. The bifunctional TNRs@PDA/Ag-based highly sensitive and duplex SERS dye detection will have enormous potential for food safety monitoring.

Oxygen-Deficient Bioceramics: Combination of Diagnosis, Therapy, and Regeneration

The journey of ceramics in medicine has been synchronized with an evolution from the first generation-alumina, zirconia, etc.-to the third -3D scaffolds. There is an up-and-coming member called oxygen-deficient or colored bioceramics, which have recently found their way through biomedical applications. The oxygen vacancy steers the light absorption toward visible and near infrared regions, making the colored bioceramics multifunctional-therapeutic, diagnostic, and regenerative. Oxygen-deficient bioceramics are capable of turning light into heat and reactive oxygen species for photothermal and photodynamic therapies, respectively, and concomitantly yield infrared and photoacoustic images. Different types of oxygen-deficient bioceramics have been recently developed through various synthesis routes. Some of them like TiO2- x , MoO3- x , and WOx have been more investigated for biomedical applications, whereas the rest have yet to be scrutinized. The most prominent advantage of these bioceramics over the other biomaterials is their multifunctionality endowed with a change in the microstructure. There are some challenges ahead of this category discussed at the end of the present review. By shedding light on this recently born bioceramics subcategory, it is believed that the field will undergo a big step further as these platforms are naturally multifunctional.

The Convenience of Polydopamine in Designing SERS Biosensors with a Sustainable Prospect for Medical Application

Polydopamine (PDA) is a multifunctional biomimetic material that is friendly to biological organisms and the environment, and surface-enhanced Raman scattering (SERS) sensors have the potential to be reused. Inspired by these two factors, this review summarizes examples of PDA-modified materials at the micron or nanoscale to provide suggestions for designing intelligent and sustainable SERS biosensors that can quickly and accurately monitor disease progression. Undoubtedly, PDA is a kind of double-sided adhesive, introducing various desired metals, Raman signal molecules, recognition components, and diverse sensing platforms to enhance the sensitivity, specificity, repeatability, and practicality of SERS sensors. Particularly, core-shell and chain-like structures could be constructed by PDA facilely, and then combined with microfluidic chips, microarrays, and lateral flow assays to provide excellent references. In addition, PDA membranes with special patterns, and hydrophobic and strong mechanical properties can be used as independent platforms to carry SERS substances. As an organic semiconductor material capable of facilitating charge transfer, PDA may possess the potential for chemical enhancement in SERS. In-depth research on the properties of PDA will be helpful for the development of multi-mode sensing and the integration of diagnosis and treatment.

Inversion of the Complex Refractive Index of Au-Ag Alloy Nanospheres Based on the Contour Intersection Method

The contour intersection method is a new method used to invert the complex refractive index of small particles. Research has yet to be reported on using this method to invert the complex refractive index of nanoparticles. This paper reports the feasibility and reliability of the contour intersection method in the inversion of the complex refractive index of nanoparticles using Au-Ag alloy nanospheres. The Mie theory and the size-dependent dielectric function are used to calculate the light scattering and absorption efficiency of Au-Ag alloy nanospheres corresponding to the complex refractive index. The complex refractive index of the particles is obtained by inversion with the contour intersection method. The backscattering efficiency constraint method is used to determine the unique solution when multiple valid solutions from the contour intersection method appear. The effects of the Au component percentage, particle size, and measurement errors on the inversion results are quantitatively analyzed. Finally, the inversion accuracy is compared and analyzed with the traditional iterative method. The results show that as long as the light scattering efficiency, light absorption efficiency, and backscattering efficiency of Au nanospheres can be measured, the accurate complex refractive index can also be calculated by inversion using the contour intersection method. The accuracy of the inversion results can be ensured when the measurement error is less than 5%. The results of inversion using the contour intersection method are better than those of the iterative methods under the same conditions. This study provides a simple and reliable inversion method for measuring the complex refractive index of Au-Ag alloy nanospheres.

Constructions of synergistic photothermal therapy antibacterial hydrogel based on polydopamine, tea polyphenols and polyvinyl alcohol and effects on wound healing in mouse

Hydrogels with significant antibacterial efficacy have enormous potential in wound healing. This work reported a series of effective antibacterial and antioxidant hydrogels based on tea polyphenols (TP), polydopamine (PDA), and polyvinyl alcohol (PVA). PDA-TP molecular chains are formed from oxidized TP and PDA. These molecular chains, which were cross-linked with PVA by cyclic freeze-thaw (FT), formed the PVA/PDA-TP hydrogel (PPTP). The number of freezing-thawing cycles and the amount of TP would affect the mechanical properties and swelling properties of hydrogel. The PPTP hydrogel exhibited high photo thermal conversion efficiency, high antibacterial efficacy, antioxidant properties, good cellular compatibility and short wound closure time. The PPTP hydrogel leaded to wound closure in approximately 10 d in a full-thickness skin defect mouse model. The preparation method of hydrogel with non-chemical cross-linked and ability of rapid high temperature generation provided a new way to apply TP to wound healing and proved that synergistic chemical and photothermal therapy can effectively inhibit resistant bacteria and accelerate wound healing.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Selective Photocatalytic Activation of Ethanol C-H and O-H Bonds over Multi-Au@SiO2/TiO2: Role of Catalyst Surface Structure and Reaction Kinetics

The chemical bond diversity and flexible reactivity of biomass-derived ethanol make it a vital feedstock for the production of value-added chemicals but result in low conversion selectivity. Herein, composite catalysts comprising SiO₂-coated single- or multiparticle Au cores hybridized with TiO₂ nanoparticles (mono- or multi-Au@SiO₂/TiO₂, respectively) were fabricated via electrostatic self-assembly. The C-H and O-H bonds of ethanol were selectively activated (by SiO₂ and TiO₂, respectively) under irradiation to form CH₃CH^•(OH) or CH₃CH₂O^• radicals, respectively. The formation and depletion kinetics of these radicals was analyzed by electron spin resonance to reveal marked differences between mono- and multi-Au@SiO₂/TiO₂. Consequently, the selectivity of these catalysts for 1,1-diethoxyethane after 6 h irradiation was determined as 81 and 99%, respectively, which was attributed to the more pronounced effect of localized surface plasmon resonance for multi-Au@SiO₂/TiO₂. Notably, only acetaldehyde was formed on a Au/TiO₂ catalyst without a SiO₂ shell. Fourier transform infrared (FTIR) spectroscopy indicated that the C-H adsorption of ethanol was enhanced in the case of multi-Au@SiO₂/TiO₂, while NH₃ temperature-programmed desorption and pyridine adsorption FTIR spectroscopy revealed that multi-Au@SiO₂/TiO₂ exhibited enhanced surface acidity. Collectively, the results of experimental and theoretical analyses indicated that the adsorption of acetaldehyde on multi-Au@SiO₂/TiO₂ was stronger than that on Au/TiO₂, which resulted in the oxidative coupling of ethanol to afford 1,1-diethoxyethane on the former and the dehydrogenation of ethanol to acetaldehyde on the latter.

Crystalline Phase Induced Raman Enhancement on Molybdenum Carbide

Crystalline phase can greatly influence the Raman enhancement on semiconductor materials. Here, we demonstrate the crystalline phase induced Raman enhancement on molybdenum carbide materials (β-Mo2C and α-MoC). From all the...

Plasmonic Oxygen Defects in MO3- x (M = W or Mo) Nanomaterials: Synthesis, Modifications, and Biomedical Applications

Nanomedicine is a promising technology with many advantages and provides exciting opportunities for cancer diagnosis and therapy. During recent years, the newly developed oxygen-deficiency transition metal oxides MO_{3- x} (M = W or Mo) have received significant attention due to the unique optical properties, such as strong localized surface plasmon resonance (LSPR) , tunable and broad near-IR absorption, high photothermal conversion efficiency, and large X-ray attenuation coefficient. This review presents an overview of recent advances in the development of MO_{3- x} nanomaterials for biomedical applications. First, the fundamentals of the LSPR effect are introduced. Then, the preparation and modification methods of MO_{3- x} nanomaterials are summarized. In addition, the biological effects of MO_{3- x} nanomaterials are highlighted and their applications in the biomedical field are outlined. This includes imaging modalities, cancer treatment, and antibacterial capability. Finally, the prospects and challenges of MO_{3- x} and MO_{3- x} -based nanomaterial for fundamental studies and clinical applications are also discussed.

A ratiometric fluorescence sensor for ascorbic acid determination based on an AND-NAND logic pair

An AND-NAND logic pair is reported based on non-purified carbon quantum dots (CDs) for ascorbic acid (AA) detection. In the logic operation, molybdenum oxide nanosheets (MoO3 NSs) and AA are used as two signal inputs. In the presence of AA, MoO3 NSs are reduced to plasmonic molybdenum oxide, which decreases the CD fluorescence intensity because of a static quenching, dynamic quenching, and internal filtration effect. Meanwhile, the AA is oxidized to dehydroascorbic acid and forms fluorescent 3-(dihydroxyethyl) furo [3,4-b] quinoxaline-1-one with o-phenylenediamine from non-purified CDs. On this basis, an AND-NAND logic pair was constructed and used as a ratiometric fluorescence sensor for highly sensitive detection of AA. The method has a wide linear range of 0.05-50 μM, and a detection limit of 34 nM. In addition, it was used to detect AA in fresh fruit. Potential applications include chemical computing, optoelectronic devices, biomedical science, and environmental monitoring. HIGHLIGHTS: 1. A ratiometric fluorescence sensor based on AND-NAND logic pair constructed by CDs and MoO3 NSs was successfully fabricated. 2. The ratiometric fluorescence sensor exhibited satisfactory linear range, high sensitivity, and good selectivity for AA. 3. The ratiometric fluorescence method was able to detect AA in fresh fruit with good results comparable to official fluorescence methods.

Phenolic-enabled nanotechnology: versatile particle engineering for biomedicine

Phenolics are ubiquitous in nature and have gained immense research attention because of their unique physiochemical properties and widespread industrial use. In recent decades, their accessibility, versatile reactivity, and relative biocompatibility have catalysed research in phenolic-enabled nanotechnology (PEN) particularly for biomedical applications which have been a major benefactor of this emergence, as largely demonstrated by polydopamine and polyphenols. Therefore, it is imperative to overveiw the fundamental mechanisms and synthetic strategies of PEN for state-of-the-art biomedical applications and provide a timely and comprehensive summary. In this review, we will focus on the principles and strategies involved in PEN and summarize the use of the PEN synthetic toolkit for particle engineering and the bottom-up synthesis of nanohybrid materials. Specifically, we will discuss the attractive forces between phenolics and complementary structural motifs in confined particle systems to synthesize high-quality products with controllable size, shape, composition, as well as surface chemistry and function. Additionally, phenolic's numerous applications in biosensing, bioimaging, and disease treatment will be highlighted. This review aims to provide guidelines for new scientists in the field and serve as an up-to-date compilation of what has been achieved in this area, while offering expert perspectives on PEN's use in translational research.

Synthesis and defect engineering of molybdenum oxides and their SERS applications

Surface-enhanced Raman scattering (SERS) spectroscopy has been developed into a cross-disciplinary analytical technology through exploring various materials' Raman vibrational modes with ultra-high sensitivity and specificity. Although conventional noble-metal based SERS substrates have achieved great success, oxide-semiconductor-based SERS substrates are attracting researchers' intensive interest due to their merits of facile fabrication, high uniformity and tunable SERS characteristics. Among all the SERS active oxide semiconductors, molybdenum oxides (MoO_x) possess exceptional advantages of high Raman enhancement factor, environmental stability, recyclable detection, etc. More interestingly, the SERS effect of the MoO_x SERS substrates may involve both the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which is determined by the stoichiometry and morphology of the material. Therefore, the focus of this review will be on two critical points: (1) synthesis and material engineering methods of the functional MoO_x material and (2) MoO_x SERS mechanism and performance evaluation. First, we review recent works on the MoO_x preparation and material property tuning approaches. Second, the SERS mechanism and performance of various MoO_x substrates are surveyed. In particular, the performance uniformity, enhancement factor and recyclability are evaluated. In the end, we discuss several challenges and open questions related to further promoting the MoO_x as the SERS substrate for monitoring extremely low trace molecules and the theory for better understanding of the SERS enhancement mechanism.

A mixed valence state Mo-based metal–organic framework from photoactivation as a surface-enhanced Raman scattering substrate

In this work, a facile method for a mixed valence state Mo-base metal-organic framework from photo activation (UV Mo-MOF) was proposed and employed as a SERS substrates with molecule enrichment property.

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.

Supercritical CO2 synthesis of Co-doped MoO3-x nanocrystals for multifunctional light utilization

Here, we demonstrate, for the first time, that Co-MoO_3-x nanocrystals (NCs) have been synthesized with the assistance of supercritical CO₂. Their unique structural features of transition-metal doping and high oxygen vacancy concentrations, lead to synchronous outstanding surface enhanced Raman scattering (SERS) detection and photothermal conversion performances.

Hot Carriers and Photothermal Effects of Monolayer MoOx for Promoting Sulfite Oxidase Mimetic Activity

Local surface plasmon resonance (LSPR)-enhanced catalysis has brought a substantial amount of opportunities across various disciplines such as photocatalysis, photodetection, and photothermal therapeutics. Plasmon-induced photothermal and hot carriers effects have also been utilized to activate the enzyme-like reactions. Compared with natural enzymes, the relatively low catalytic performance of nanozymes severely hampered the potential applications in the field of biomedicine. For these issues mentioned above, herein, we demonstrate a highly efficient sulfite oxidase (SuO_x) mimetic performance of plasmonic monolayer MoO_x (ML-MoO_x) upon LSPR excitation. We also established that the considerable photothermal effect and the injection of hot carriers induced by LSPR are responsible for promoting the SuO_x activity of ML-MoO_x. The high transient local temperature on the surface of ML-MoO_x generated by the photothermal effect facilitates to impact the reaction velocity and feed the SuO_x-like activity, while the generation of hot carriers which are suggested as predominant effects catalyzes the oxidation of sulfite to sulfate through significantly decreasing the activation energy for the SuO_x-like reaction. These investigations present a contribution to the basic understanding of plasmon-enhanced enzyme-like reaction and provided an insight into the optimization of the SuO_x mimetic performance of nanomaterials.

Plasmonic Molybdenum Tungsten Oxide Hybrid with Surface-Enhanced Raman Scattering Comparable to that of Noble Metals

The surface-enhanced Raman scattering (SERS) research is in full swing owing to its high sensitivity and high selectivity; however, the substrates with superexcellent performance for SERS are largely confined to noble metals (Au, Ag, etc.). Although the SERS active substrates have been extended to semiconductors and transition metals, it is frustrating that their sensitivities are insufficient for widespread practical application. Here, we report the plasmonic molybdenum tungsten oxide (MWO) hybrid nanomaterials (NMs), which can be used as high-performance substrates with SERS comparable to that of noble metals. MWO NMs can achieve the trace detection of rhodamine 6G (R6G), basic fuchsin (BF), and oil red O (ORO). The detection limit concentration for R6G is 10^-8 M, with the maximum enhancement factor of up to 6.09 × 10⁷. The superexcellent SERS performance was attributed to the cooperative enhancement effect of electromagnetic (EM) enhancement mechanism and the charge transfer (CT) mechanism. Moreover, in the proposed system, the EM and CT contribution was distinguished by employing poly(vinylpyrrolidone) (PVP), which serves as a barrier layer to prevent the CT process from MWO NMs to R6G. These remarkable MWO NMs can be obtained with a facile method, and this research provides new insight into non-noble metal based SERS substrate.