Raman spectroscopy and its plasmon-enhanced counterparts: A toolbox to probe protein dynamics and aggregation.

Protein unfolding and aggregation are often correlated with numerous diseases such as Alzheimer's, Parkinson's, Huntington's, and other debilitating neurological disorders. Such adverse events consist of a plethora of competing mechanisms, particularly interactions that control the stability and cooperativity of the process. However, it remains challenging to probe the molecular mechanism of protein dynamics such as aggregation, and monitor them in real-time under physiological conditions. Recently, Raman spectroscopy and its plasmon-enhanced counterparts, such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), have emerged as sensitive analytical tools that have the potential to perform molecular studies of functional groups and are showing significant promise in probing events related to protein aggregation. We summarize the fundamental working principles of Raman, SERS, and TERS as nondestructive, easy-to-perform, and fast tools for probing protein dynamics and aggregation. Finally, we highlight the utility of these techniques for the analysis of vibrational spectra of aggregation of proteins from various sources such as tissues, pathogens, food, biopharmaceuticals, and lastly, biological fouling to retrieve precise chemical information, which can be potentially translated to practical applications and point-of-care (PoC) devices. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > Diagnostic Nanodevices Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Photoactivated plasmonic nanohybrid fibers with prolonged trapping of excited charge carriers for SERS analysis of biomolecules.

The quest to enhance Raman spectroscopic signals through the rational design of plasmonic substrates has enabled the detection and characterization of pharmaceutically important molecules with low scattering cross-sections, such as amino acids and proteins, and is helping in making forays into the diverse field of biomedical sciences. This work presents a simple strategy for synthesizing silver nanoparticles-incorporated alumina nanofibers (Ag-AlNFs) utilizing controlled microwave synthesis for enhancing the surface-enhanced Raman chemical enhancement factor through photo-induced charge accumulation at the plasmonic-dielectric interface. The plasmonic-dielectric fibers serve as excellent charge carrier trappers, as evident from the ultrafast transient absorption spectroscopy studies. Apart from chemical enhancement, the increase in electronic surface charge also enables the protein disulfide bonds to capture these electrons and form a transient disulfide electron adduct radical, which converts to free thiol radical on dissociation. This allows protein molecules to bind to the nanoparticle's surface with the favorable silver thiol bond leading to greater surface affinity and larger SERS enhancement. The proposed Ag-AlNFs represent a cost-effective material that can be potentially used to probe biological systems in a label-free manner by photoactivating the SERS substrate for obtaining higher enhancement factors.

Emergence of Raman Spectroscopy as a Probing Tool for Theranostics.

Although medical advances have increased our grasp of the amazing morphological, genetic, and phenotypic diversity of diseases, there are still significant technological barriers to understanding their complex and dynamic character. Specifically, the complexities of the biological systems throw a diverse set of challenges in developing efficient theranostic tools and methodologies that can probe and treat pathologies. Among several emerging theranostic techniques such as photodynamic therapy, photothermal therapy, magnetic resonance imaging, and computed tomography, Raman spectroscopy (RS) is emerging as a promising tool that is a label-free, cost-effective, and non-destructive technique. It can also provide real-time diagnostic information and can employ multimodal probes for detection and therapy. These attributes make it a perfect candidate for the analytical counterpart of the existing theranostic probes. The use of biocompatible nanomaterials for the fabrication of Raman probes provides rich structural information about the biological molecules, cells, and tissues and highly sensitive information down to single-molecule levels when integrated with advanced RS tools. This review discusses the fundamentals of Raman spectroscopic tools such as surface-enhanced Raman spectroscopy and Resonance Raman spectroscopy, their variants, and the associated theranostic applications. Besides the advantages, the current limitations, and future challenges of using RS in disease diagnosis and therapy have also been discussed.

Ion-Mediated Protein Stabilization on Nanoscopic Surfaces.

The emergence of nanoparticles in biomedical applications has made their interactions with proteins inevitable. Nanoparticles conjugated with proteins and peptide-based constructs form an integral part of nanotherapeutics and have recently shown promise in treating a myriad of diseases. The proper functioning of proteins is critical to achieve their biological functions. However, interface issues result in the denaturation of proteins, and the loss of orientation and steric hindrance can adversely affect the function of the conjugate. Furthermore, surface-induced denaturation also triggers protein aggregation, resulting in amyloid-like species. Understanding the mechanistic underpinnings of protein-nanoparticle interactions and controlling their interfacial characteristics are critical and challenging due to the complex nature of the conjugates. In this milieu, we demonstrate that ionic liquids can be suitable candidates for stabilizing protein-nanoparticle interactions by virtue of their excellent protein-preserving properties. We also probe the previously unexplored mechanism of ion-mediated stabilization of the protein molecules on the nanoparticle surface. The protein-nanoparticle conjugates consist of lysozyme and choline-based ionic liquids characterized by optical and electron microscopy techniques combined with surface-sensitive plasmon-enhanced Raman spectroscopy. Furthermore, atomistic molecular dynamics simulations of the conjugates delineate interfacial interactions of the protein molecules and the modulation by the ions, particularly the conformational changes and the dynamic correlation when the protein and specific ionic liquid molecules are adsorbed on the nanoparticle surface. The combined experimental and computational studies showed the synergistic behavior of the ions of the ionic liquids, specifically the orientation and coverage of the anions aided by the cations to control the surface interactions and hence the overall protein stability. These studies pave the way for using ionic liquids, particularly their biocompatible counterparts in nanoparticle-based complexes, as stabilizing agents for biomedical applications.

Toward Continuous Breath Monitoring on a Mobile Phone Using a Frugal Conducting Cloth-Based Smart Mask.

A frugal humidity sensor that can detect changes in the humidity of exhaled breath of individuals has been fabricated. The sensor comprises a humidity-sensitive conducting polymer that is in situ formed on a cloth that acts as a substrate. Interdigitated silver electrodes were screen-printed on the modified cloth, and conducting threads connected the electrodes to the measurement circuit. The sensor's response to changing humidity was measured as a voltage drop across the sensor using a microcontroller. The sensor was capable of discerning between fast, normal, and slow breathing based on the response time. A response time of ∼1.3 s was observed for fast breathing. An Android-based mobile application was designed to collect sensor data via Bluetooth for analysis. A time series classification algorithm was implemented to analyze patterns in breathing. The sensor was later stitched onto a face mask, transforming it into a smart mask that can monitor changes in the breathing pattern at work, play, and sleep.

Spatial reorganization of analytes in charged aqueous microdroplets.

Imprinted charged aqueous droplets of micrometer dimensions containing spherical gold and silver nanoparticles, gold nanorods, proteins and simple molecules were visualized using dark-field and transmission electron microscopies. With such studies, we hoped to understand the unusual chemistry exhibited by microdroplets. These droplets with sizes in the range of 1-100 µm were formed using a home-built electrospray source with nitrogen as the nebulization gas. Several remarkable features such as mass/size-selective segregation and spatial localization of solutes in nanometer-thin regions of microdroplets were visualized, along with the formation of micro-nano vacuoles. Electrospray parameters such as distance between the spray tip and surface, voltage and nebulization gas pressure influenced particle distribution within the droplets. We relate these features to unusual phenomena such as the enhancement of rates of chemical reactions in microdroplets.

Role of Zinc Oxide in the Compounding Formulation on the Growth of Nonstoichiometric Copper Sulfide Nanostructures at the Brass-Rubber Interface.

Tire technology has evolved substantially by the introduction of brass-coated steel cords (BCSCs) in radial tires. The durability of radial tires is dependent on the integrity of the brass-rubber interface composed predominantly of nonstoichiometric copper sulfide (Cu2-x S, where x = 1 to 2) nanostructures whose morphology and characteristics are dependent upon the crucial rubber additive, ZnO. Its higher concentration impacts environmental sustainability, while at lower levels, there is insufficient bonding between steel and the rubber thus affecting tire's safety. This brings in the need for an optimum ZnO concentration to be used in radial tires and is thus the theme of the present work. The changes in the properties of interfacial nanostructures such as morphology, thickness, crystallinity, and chemical composition were studied at various ZnO concentrations. We adopted our previously reported methodology, the "brass mesh experiment", to investigate the thickness of nanostructures at varied ZnO concentrations using transmission electron microscopy (TEM). Significant results were obtained from field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman imaging and X-ray photoelectron spectroscopy (XPS). In conjunction with a more practical experimental technique, namely the measurement of pull-out force (POF), it has been concluded that 9 parts per hundred rubber (PHR) ZnO is essential for the optimum growth of nanostructures and is considered to be the optimum for the composition studied. We believe that the scientific approach outlined in the manuscript would help the tire- and the material science communities to widen the knowledge of understanding sustainability in tire industries. It is estimated that the optimization presented here can save $400-450 million for the tire industry and 2.4 million tons of ZnO per year.

Gold cluster-loaded dendritic nanosilica: single particle luminescence and catalytic properties in the bulk.

We report a hybrid material in which surface anchoring-induced enhanced luminescence of AuQC@BSA clusters on high surface area dendritic fibrous nanosilica of 800 nm diameter enabled their luminescence imaging at a single particle level. The photophysical and structural properties of the hybrid material were characterized by various spectroscopic and microscopic techniques. Concomitant imaging using scattering and luminescence of such mesostructures and their response to analytes have been used to develop a chemical sensor. The hybrid material was found to be catalytically active in silane to silanol conversion, and 100% conversion was observed in 4 h when the reaction was carried out at 30 °C in the presence of light. Such materials at submicron dimensions with enhanced surface area, emission in the solid state along with a high quantum yield of 12% in water along with enhanced scattering, and surface functionalities present numerous benefits for the creation of multifunctional materials.

2D-Molybdenum Disulfide-Derived Ion Source for Mass Spectrometry.

Generation of current or potential at nanostructures using appropriate stimuli is one of the futuristic methods of energy generation. We developed an ambient soft ionization method for mass spectrometry using 2D-MoS2, termed streaming ionization, which eliminates the use of traditional energy sources needed for ion formation. The ionic dissociation-induced electrokinetic effect at the liquid-solid interface is the reason for energy generation. We report the highest figure of merit of current generation of 1.3 A/m2 by flowing protic solvents at 22 µL/min over a 1 × 1 mm2 surface coated with 2D-MoS2, which is adequate to produce continuous ionization of an array of analytes, making mass spectrometry possible. Weakly bound ion clusters and uric acid in urine have been detected. Further, the methodology was used as a self-energized breath alcohol sensor capable of detecting 3% alcohol in the breath.

Atom transfer between precision nanoclusters and polydispersed nanoparticles: a facile route for monodisperse alloy nanoparticles and their superstructures.

Reactions between atomically precise noble metal nanoclusters (NCs) have been studied widely in the recent past, but such processes between NCs and plasmonic nanoparticles (NPs) have not been explored earlier. For the first time, we demonstrate spontaneous reactions between an atomically precise NC, Au25(PET)18 (PET = 2-phenylethanethiol), and polydispersed silver NPs with an average diameter of 4 nm and protected with PET, resulting in alloy NPs under ambient conditions. These reactions were specific to the nature of the protecting ligands as no reaction was observed between the Au25(SBB)18 NC (SBB = 4-(tert-butyl)benzyl mercaptan) and the very same silver NPs. The mechanism involves an interparticle exchange of the metal and ligand species where the metal-ligand interface plays a vital role in controlling the reaction. The reaction proceeds through transient Au25-xAgx(PET)n alloy cluster intermediates as observed in time-dependent electrospray ionization mass spectrometry (ESI MS). High-resolution transmission electron microscopy (HRTEM) analysis of the resulting dispersion showed the transformation of polydispersed silver NPs into highly monodisperse gold-silver alloy NPs which assembled to form 2-dimensional superlattices. Using NPs of other average sizes (3 and 8 nm), we demonstrated that size plays an important role in the reactivity as observed in ESI MS and HRTEM.

Ambient electrospray deposition Raman spectroscopy (AESD RS) using soft landed preformed silver nanoparticles for rapid and sensitive analysis.

We introduce a technique called ambient electrospray deposition Raman spectroscopy (AESD RS) for rapid and sensitive surface-enhanced Raman scattering (SERS) based detection of analytes using a miniature Raman spectrometer. Using electrospray, soft landing of preformed silver nanoparticles (AgNPs) was performed for 30-40 seconds for different concentrations of analytes deposited on conducting glass slides. Using AESD RS, SERS signals were collected within 4-6 minutes, including sample preparation. Transmission electron microscopy (TEM) and dark-field microscopy (DFM) were used to characterize the preformed AgNPs before and after electrospray. We achieved the nanomolar and micromolar detection of p-mercaptobenzoic acid (p-MBA) and 2,4-dinitrotoluene (2,4-DNT), respectively. In this work, 0.3 µL of preformed AgNPs were used, which is ∼33 times less in volume than the quantity needed for conventional SERS. Quantitation of unknown concentration of analytes was also possible. A similar amount of electrosprayed AgNPs was utilized to characterize Escherichia coli (E. coli) bacteria of different concentrations. Viability of bacteria was tested using fluorescence microscopic imaging. Besides reduced analysis time and improved reproducibility of the data in every analysis, which is generally difficult in SERS, the amount of AgNPs required is an order of magnitude lower in this method. This method could also be used to probe the real-time changes in molecular and biological species under ambient conditions.

Highly Sensitive As3+ Detection Using Electrodeposited Nanostructured MnOx and Phase Evolution of the Active Material during Sensing.

A simple, one-step electrodeposition approach has been used to fabricate MnOx on an indium-doped tin oxide substrate for highly sensitive As3+ detection. We report an experimental limit of detection of 1 ppb through anodic stripping voltammetry with selectivity to As3+ in the presence of 10 times higher concentrations of several metal ions. Additionally, we report the simultaneous phase evolution of active material occurring through multiple stripping cycles, wherein MnO/Mn2O3 eventually converts to Mn3O4 as a result of change in the oxidation states of manganese. This occurs with concomitant changes in morphology. Change in the electronic property (increased charge transfer resistance) of the material due to sensing results in an eventual decrease in sensitivity after multiple stripping cycles. In a nutshell, this paper reports stripping-voltammetry-induced change in morphology and phase of as-prepared Mn-based electrodes during As sensing.

Confining an Ag10 Core in an Ag12 Shell: A Four-Electron Superatom with Enhanced Photoluminescence upon Crystallization.

We introduce a cluster coprotected by thiol and diphosphine ligands, [Ag22(dppe)4(2,5-DMBT)12Cl4]2+ (dppe = 1,2-bis(diphenylphosphino)ethane; 2,5-DMBT= 2,5-dimethylbenzenethiol), which has an Ag10 core encapsulated by an Ag12(dppe)4(2,5-DMBT)12Cl4 shell. The Ag10 core comprises two Ag5 distorted trigonal bipyramidal units and is uncommon in Au and Ag nanoclusters. The electrospray ionization mass spectrum reveals that the cluster is divalent and contains four free electrons. An uncommon crystallization-induced enhancement of emission is observed in the cluster. The emission is weak in the solution and amorphous states. However, it is enhanced 12 times in the crystalline state compared to the amorphous state. A detailed investigation of the crystal structure suggests that well-arranged C-H···π and π···π interactions between the ligands are the major factors for this enhanced emission. Further, in-depth structural elucidation and density functional theory calculations suggest that the cluster is a superatom with four magic electrons.

Appearance of SERS activity in single silver nanoparticles by laser-induced reshaping.

We report simultaneous plasmonic scattering and Raman spectroscopic observations of single citrate capped silver nanoparticles (AgNPs) which exhibit surface enhanced Raman scattering (SERS) upon meeting specific conditions induced by laser (532 nm) exposure. We show that nanoparticles which are not initially SERS active become SERS active by laser-induced reshaping/reorientation. A set-up developed for these observations enabled in situ high speed time-lapse characterization using plasmonic and Raman spectroscopies in conjunction with dark-field microscopy (DFM). Changes in the AgNPs were confirmed by monitoring plasmonic scattering spectra and DFM images. Time-lapse observations have shown that laser-induced changes in the plasmonic properties of AgNPs resulted in the appearance of SERS. Spectral matching between plasmon resonance and downward molecular vibronic transitions for molecules adsorbed on the surface of plasmonic nanomaterials is attributed to the nanoparticle SERS. We have further shown that the release of silver ions by silver nanoparticles can be the probable reason for their plasmonic changes. Gold nanoparticles inert to such mild (850 µW, 532 nm) laser-induced changes do not exhibit the appearance of SERS.

Polymorphism of Ag29(BDT)12(TPP)43- cluster: interactions of secondary ligands and their effect on solid state luminescence.

We present the first example of polymorphism (cubic & trigonal) in single crystals of an atomically precise monolayer protected cluster, Ag29(BDT)12(TPP)43-. We demonstrate that C-Hπ interactions of the secondary ligands (TPP) are dominant in a cubic lattice compared to a trigonal lattice, resulting in a greater rigidity of the structure, which in turn, results in a higher luminescence efficiency in it.

Rapid reaction of MoS2 nanosheets with Pb2+ and Pb4+ ions in solution.

Understanding the chemical changes happening to nanostructures during a process is vital in selecting them for applications. Here, we investigated the difference in the reactivity of the bulk and nanoscale forms of molybdenum disulfide (MoS2) in solution with lead ions (Pb2+ and Pb4+) as probes, at room temperature. While the bulk form did not show any reactivity in the experimental timescale, the two-dimensional (2D) nanoscale form showed not only reactivity but also quite rapid kinetics that resulted in the formation of distinct products, principally PbMoO4 with anion substitution, in a few seconds. Depending on the charge state of the cation, and the pH of the reaction mixture, two different kinds of morphologies of the same reaction product were formed. Furthermore, we demonstrate that this unusual reactivity of the MoS2 nanosheets (NSs) was retained in its supported form and hence, such supported materials can be effective for the abstraction of toxic lead from water, with fast kinetics.