Challenges and opportunities for SERS in the infrared: materials and methods

In the wake of a global, heightened interest towards biomarker and disease detection prompted by the SARS-CoV-2 pandemic, surface enhanced Raman spectroscopy (SERS) positions itself again at the forefront of biosensing innovation. But is it ready to move from the laboratory to the clinic? This review presents the challenges associated with the application of SERS to the biomedical field, and thus, to the use of excitation sources in the near infrared, where biological windows allow for cell and through-tissue measurements. Two main tackling strategies will be discussed: (1) acting on the design of the enhancing substrate, which includes manipulation of nanoparticle shape, material, and supramolecular architecture, and (2) acting on the spectral collection set-up. A final perspective highlights the upcoming scientific and technological bets that need to be won in order for SERS to stably transition from benchtop to bedside.

Arrays of Ag-nanoparticles decorated TiO2 nanotubes as reusable three-dimensional surface-enhanced Raman scattering substrates for molecule detection

Three-dimensional surface-enhanced Raman scattering (SERS) substrates usually provide more hot spots in the excitation light beam and higher sensitivity when compared with the two-dimensional counterpart. Here a simple approach is presented for the fabrication of arrays of Ag-nanoparticles decorated TiO2 nanotubes. Arrays of ZnO nanorods were fabricated in advance by a hydrothermal method. Then TiO2 nanotube arrays were achieved by immersing the arrays of ZnO nanorods in an aqueous solution of (NH4)2TiF6 for 1.5 h. Vertically aligned TiO2 nanotube arrays were modified with dense Ag nanoparticles by Ag mirror reaction. High density of Ag nanoparticles decorated on the fabricated TiO2 nanotubes provide plenty of hotspots for Raman enhancement. In addition, the fabricated array of Ag nanoparticles modified TiO2 nanotubes can serve as a reusable SERS substrate because of the photocatalytic activity of the TiO2 nanotubes. The SERS substrate adsorbed with analyte molecules can realize self-cleaning in deionized water after UV irradiation for 2.5 h. The sensitivity of the fabricated SERS substrate was investigated by the detection of organic dye molecules. The detectable concentration limits of rhodamine 6G (R6G), malachite green (MG) and methylene blue (MB) were found to be 10−12 M, 10−9 M and 10−8 M, respectively. The enhancement factor (EF) of the three-dimensional SERS substrate was estimated to be as high as ∼1.4×108. Therefore, the prepared Ag nanoparticles modified TiO2 nanotube arrays have promising potentials to be applied to rapid and trace SERS detection of organic chemicals.

Rapid and simultaneous detection of multiple illegal additives in feed and food by SERS with reusable Cu2O-Ag/AF-C3N4 substrate

Illegal additives can bring the economic benefit, resulting in the continuous irregularities in the use of illegal additives. In this study, a method for rapid, sensitive, and simultaneous detection of multiple illegal additives including enrofloxacin, malachite green, nitrofurazone, and Sudan Ⅰ in feed and food samples by surface-enhanced Raman spectroscopy (SERS) with Cu₂O-Ag/AF-C₃N₄ composite substrate was developed. A Cu₂O-Ag/AF-C₃N₄ composite substrate was prepared by reacting Cu₂O modified by AF-C₃N₄ nanosheets with AgNO₃ solution. The substrate has a limit of detection (LOD) of 1.29 × 10^-6 mg/L, a good linear relationship of between 10^-6 and 10^-2 mg/L, and an R² value of 0.95 for Rhodamine B detection. Furthermore, the substrate showed high uniformity and reproducibility, with relative standard deviations (RSD) of 6.74% and 4.85%, respectively. Adding AF-C₃N₄ nanosheets not only increased the enhancement effect of the substrate, which was 4.4 times of that before addition, but also endowed it with good self-cleaning characteristics owing to its excellent photocatalytic activity. The substrate can be reused, with over 80% of the original Raman signal strength remaining after four repeat uses. The SERS based on the above substrate was used to detect the illegal additives, the LOD of enrofloxacin, malachite green, nitrofurazone, and Sudan Ⅰ can reach 4.67 × 10^-4 mg/L, 2.57 × 10^-5 mg/L, 5.7 × 10^-7 mg/L and 6.92 × 10^-5 mg/L. The results reveal that this substrate has great application potential in feed and food safety.

Structurally Disordered RuO2 Nanosheets with Rich Oxygen Vacancies for Enhanced Nitrate Electroreduction to Ammonia

Electrochemical reduction of nitrate pollutants to ammonia has emerged as an attractive alternative for ammonia synthesis. Currently, many strategies have been developed for enhancing nitrate reduction to ammonia (NRA) efficiency, but the influence of the degree of structural disorder is still unexplored. Here, carbon-supported RuO₂ nanosheets with adjustable crystallinity are synthesized by a facile molten salt method. The as-synthesized amorphous RuO₂ displays high ammonia Faradaic efficiency (97.46 %) and selectivity (96.42 %), greatly outperforming the crystalline counterparts. The disordered structure with abundant oxygen vacancies is revealed to modulate the d-band center and hydrogen affinity, thus lowering the energy of the potential-determining step (NH₂ *→NH₃ *).

Cancer Stem Cell DNA Enabled Real-Time Genotyping with Self-Functionalized Quantum Superstructures-Overcoming the Barriers of Noninvasive cfDNA Cancer Diagnostics

Cancer diagnosis and determining its tissue of origin are crucial for clinical implementation of personalized medicine. Conventional diagnostic techniques such as imaging and tissue biopsy are unable to capture the dynamic tumor landscape. Although circulating tumor DNA (ctDNA) shows promise for diagnosis, the clinical relevance of ctDNA remains largely undetermined due to several biological and technical complexities. Here, cancer stem cell-ctDNA is used to overcome the biological complexities like the inability for molecular analysis of ctDNA and dependence on ctDNA concentration rather than the molecular profile. Ultrasensitive quantum superstructures overcome the technical complexities of trace-level detection and rapid diagnosis to detect ctDNA within its short half-life. Activation of multiple surface enhanced Raman scattering mechanisms of the quantum superstructures achieved a very high enhancement factor (1.35 × 10¹¹ ) and detection at ultralow concentration (10^-15 M) with very high reliability (RSD: 3-12%). Pilot validation with clinical plasma samples from an independent validation cohort achieved a diagnosis sensitivity of ≈95% and specificity of 83%. Quantum superstructures identified the tissue of origin with ≈75-86% sensitivity and ≈92-96% specificity. With large scale clinical validation, the technology can develop into a clinically useful liquid biopsy tool improving cancer diagnostics.

Tracking the Evolution of Metastasis with Self-Functionalized 3D Nanoprobes

Despite recent advances in cancer treatment, metastasis is the cause of mortality in 90% of cancer cases. It has now been well-established that dissemination of cancer cells to distant sites occurs very early during tumorigenesis, resulting in the minimal effect of surgical or chemotherapeutic treatments after the detection of metastasis. The underlying reason for this challenge is mostly due to the limited understanding of molecular mechanisms of the metastasis cascade, particularly related to metastatic traits. Therefore, there is an urgent need to investigate this currently invisible evolution of metastasis. The tracking of metastasis evolution has not been addressed yet. Here, we introduce, for the first time, a synchronous approach to unveil the molecular mechanisms of the metastasis cascade. As cancer stem cells (CSCs) demonstrate cancer initiation, drug resistance, metastasis, and tumor relapse and can exist in a quasi-intermediate epithelial-mesenchymal transition state, the tumor-initiating events during a CSCs metamorphosis were monitored with single-cell sensitivity. Because of the invasive and resistive properties of the metastable intermediate CSCs, investigation of the molecular profiles of the quasi-intermediate CSCs was necessary for the detection of metastasis dissemination. For this purpose, the ultrasensitive technique of surface-enhanced Raman scattering (SERS) was adopted. Titanium-based, biocompatible three-dimensional (3D) nanoprobes that were synthesized for multiphoton ionization achieved a substantial SERS enhancement of ∼80-fold due to the oxygen vacancy-enriched composition of the nanoprobes. The 3D interconnected complex nanoarchitecture of the nanoprobes enabled us to entrap the nonadherent CSCs of three metastatic cancer cell lines (triple negative breast adenocarcinoma (MDAMB231), human Caucasian colon adenocarcinoma (COLO 205), and cervical adenocarcinoma (HeLa)─all very aggressive forms of cancer). The nanoprobes not only promoted the CSC proliferation to successfully attain the quasi-intermediate states but also monitored its reprogramming into a cancer cell state. The nanoprobes substantially amplified weak intracellular Raman signals to capture the molecular events during a CSC transformation. The detection of cancer was achieved with 100% accuracy. We experimentally demonstrated that the molecular signatures of CSC reprogramming are cancer-type specific. This observation enabled us to identify the origin of metastasis with 100% accuracy, providing more clarity on the relatively unknown quasi-intermediate states. This first demonstration of CSC-based tracking of metastasis evolution has the potential to provide an insightful perspective of tumorigenesis that could be useful in cancer diagnosis and prognosis as well as in the monitoring of therapeutic interventions.

Charge-Transfer Resonance and Surface Defect-Dominated WO3 Hollow Microspheres as SERS Substrates for the miRNA 155 Assay

Chemical enhancement with charge transfer (CT) between the adsorbed Raman molecule and the semiconductor mainly contributed to semiconductor surface-enhanced Raman scattering (SERS). In this work, a three-dimensional (3D) WO₃ hollow microsphere is first developed as a SERS-active substrate. This 3D WO₃ has a smaller band gap and rich surface defects compared with flake WO₃. Interestingly, these properties in the WO₃ hollow microspheres lead to an increase in charge transfer, which causes a strong CT interaction between the substrate-Raman molecule interfaces, resulting in a large SERS enhancement. The 3D WO₃ showed an excellent SERS performance with an enhancement factor (EF) of 1.6 × 10⁶. Finally, a SERS biosensor is constructed based on the above-mentioned semiconductor materials, which can be used for the sensitive detection of miRNA 155 with a limit of detection (LOD) of 0.18 fM by employing a catalytic hairpin assembly (CHA) strategy. This work provides important guidance for semiconductor topography design to improve the SERS performance, supplying a new strategy for biomolecular analysis and disease diagnosis.

Modulating Mechanism of the LSPR and SERS in Ag/ITO Film: Carrier Density Effect

Herein, we fabricated a uniform and dispersible Ag/indium tin oxide (ITO) cosputtered film on a two-dimensional ordered polystyrene template and observed distinct localized surface plasmon resonance (LSPR) properties that can be tuned by changing the doping level. The increase in the optical band gap is due to the variation in the metallic Ag content, which can effectively change the accumulation of free electrons in the conduction band, in addition to the near-IR absorbance. Surface-enhanced Raman scattering (SERS) was used to monitor the variations in the band gap and transfer of electrons, which causes variations in the SERS intensity. The presented research provides new insights into the relationships between the carrier density and maximum absorption wavelength, band gap distribution, and charge transfer process. This is the first study on the influence of the carrier density on the properties of Ag/ITO cosputtered films and suggests practical applications of these films.

Prospects in interfaces of biomolecule DNA and nanomaterials as an effective way for improvising surface enhanced Raman scattering: A review

Surface Enhanced Raman Scattering (SERS) is a field of research that has shown promising application in the analysis of various substrate molecules by means of rough metallic surfaces. In directing the enhancement of substrate molecules in micro and nano-molar concentrations, plasmonic coupling of metal nanoparticles (NPs), morphology of metal NPs and the closely arrangement of rough metal surfaces that produces 'hot spots' can effectively increase the so-called enhancement factor (EF) that will be applicable in various fields. As the mechanistic aspects are still not clear, research has been triggered all over the world for the past two decades to have a clear understanding in chemical and electromagnetic effects. As the reproducibility of intensity of signals at low concentrations of probe molecules is of a big concern, metal NPs with various scaffolds were prepared and recently bio-molecule, DNA has been studied and showed promising advantages. This review first time highlights metal NPs with DNA interface as an effective rough metallic surface for SERS with high intensity and also with better reproducibility. Based on this review, similar kinds of scaffolds like DNA can be used to further analyze SERS activities of various metal NPs with different morphologies to have high intense signals at low concentrations of probe molecules.

Prediction of Cancer Stem Cell Fate by Surface-Enhanced Raman Scattering Functionalized Nanoprobes

Cancer stem cells (CSCs) are the fundamental building blocks of cancer dissemination, so it is desirable to develop a technique to predict the behavior of CSCs during tumor initiation and relapse. It will provide a powerful tool for pathological prognosis. Currently, there exists no method of such prediction. Here, we introduce nickel-based functionalized nanoprobe facilitated surface enhanced Raman scattering (SERS) for prediction of cancer dissemination by undertaking CSC-based surveillance. SERS profiling of CSCs of various cell lines (breast cancer, cervical cancer, and lung cancer) was compared with their cancer counterparts for the prediction of prognosis, with statistical significance of single-cell sensitivity. The single-cell sensitivity is critical as even a few CSCs are capable of initiating a tumor. Intermediate states of CSC transmutation to cancer cells and its reverse were monitored, and nanoprobe-assisted SERS profiling was undertaken. We experimentally demonstrated that the quasi-intermediate CSC states have dissimilar profiles during the transformation from cancer to CSC and vice versa enabling statistical differentiation without ambiguity. It was also observed that molecular signatures of these opposite pathways are cancer-type specific. This observation provided additional clarity to the current understanding of relatively unfamiliar quasi-intermediate states; making it possible to predict CSC dissemination for variety of cancers with ∼99% accuracy. Nano probe-based prediction of CSC fate is a powerful prediction tool for ultrasensitive prognosis of malignancy in a complex environment. Such CSC-based cancer prognosis has never been proposed before. This prediction technique has potential to provide insights for cancer diagnosis and prognosis as well as for obtaining information instrumental in designing of meaningful CSC-based cancer therapeutics.

A Blinking Mesoporous TiO2-x Composed of Nanosized Anatase with Unusually Long-Lived Trapped Charge Carriers

A mesoporous TiO_2-x material comprised of small, crystalline, vacancy-rich anatase nanoparticles (NPs) shows unique optical, thermal, and electronic properties. It is synthesized using polymer-derived mesoporous carbon (PDMC) as a template. The PDMC pores serve as physical barriers during the condensation and pyrolysis of a titania precursor, preventing the titania NPs from growing beyond 10 nm in size. Unlike most titania nanomaterials, during pyrolysis the NPs undergo no transition from the anatase to rutile phase and they become catalytically active reduced TiO_2-x . When exposed to a slow electron beam, the NPs exhibit a charge/discharge behavior, lighting up and fading away for an average period of 15 s for an extended period of time. The NPs also show a 50 nm red-shift in their UV/Vis absorption and long-lived charge carriers (electrons and holes) at room temperature in the dark, even long after UV irradiation. The NPs as photocatalysts show a good activity for CO₂ reduction.