Nirmal Goswami

Copper Quantum Clusters in Protein Matrix: Potential Sensor of Pb2+ Ion

A one-pot synthesis of extremely stable, water-soluble Cu quantum clusters (QCs) capped with a model protein, bovine serum albumin (BSA), is reported. From matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, we assign the clusters to be composed of Cu(5) and Cu(13) cores. The QCs also show luminescence properties having excitation and emission maxima at 325 and 410 nm, respectively, with a quantum yield of 0.15, which are found to be different from that of protein alone in similar experimental conditions. The quenching of luminescence of the protein-capped Cu QCs in the presence of very low hydrogen peroxide concentration (approximately nanomolar, or less than part-per-billion) reflects the efficacy of the QCs as a potential sensing material in biological environments. Moreover, as-prepared Cu QCs can detect highly toxic Pb(2+) ions in water, even at the part-per-million level, without suffering any interference from other metal ions.

Luminescent Metal Nanoclusters with Aggregation-Induced Emission

Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials because of their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in the biomedical field because of their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspective, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss the physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

Ag7Au6: A 13-Atom Alloy Quantum Cluster†

An alloy cluster containing a 13-atom core, with a composition Ag 7 Au 6 (H 2 MSA) 10 (H 2 MSA=mercaptosuccinic acid) was synthesized from silver clusters by a galvanic exchange reaction. The clusters were characterized by several spectroscopic and microscopic methods. The alloy cluster shows luminescence with a quantum yield of 3.5×10 -2 at room temperature. Theoretical calculations for Ag 7 Au 6 (SCH 3 ) 10 suggest a distorted icosahedral core.

Recent advances in the synthesis, characterization, and biomedical applications of ultrasmall thiolated silver nanoclusters

With ultrasmall particle sizes of ∼1 nm, thiolate-protected silver nanoclusters (or thiolated Ag NCs) have recently emerged as an attractive frontier of nanoparticle research because of their unique molecular-like properties, such as well-defined molecular structures, HOMO–LUMO transitions, quantized charging, and strong luminescence. Such intriguing physicochemical properties have made thiolated Ag NCs a new class of promising theranostic agents for a wide spectrum of biomedical applications, such as bioimaging, antimicrobial agents, and disease diagnostics and therapy. In turn, the promising applications of thiolated Ag NCs have also fuelled the cluster community to develop more efficient strategies to synthesize high-quality Ag NCs with well-defined size, structure, and surface. In this review article, we first survey recent advances in developing efficient synthetic strategies for thiolated Ag NCs, highlighting the underlying chemistry that makes the delicate control of their sizes and surfaces possible. In the second section, we discuss recent advances in characterization techniques for ultrasmall thiolated Ag NCs, including their physical, chemical, and biological properties. The emerging characterization techniques are central to the development of cluster chemistry. In the last section, we highlight some examples demonstrating the vast possibilities of thiolated Ag NCs for biomedical applications. We conclude this review article by pointing out some challenging issues related to thiolated Ag NCs, and hopefully these can encourage more concerted efforts on their study from the research communities of cluster chemistry, noble metal chemistry, biology, biomedicine, etc.

Protein-directed synthesis of NIR-emitting, tunable HgS quantum dots and their applications in metal-ion sensing.

The development of luminescent mercury sulfide quantum dots (HgS QDs) through the bio-mineralization process has remained unexplored. Herein, a simple, two-step route for the synthesis of HgS quantum dots in bovine serum albumin (BSA) is reported. The QDs are characterized by UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, luminescence, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), circular dichroism (CD), energy dispersive X-ray analysis (EDX), and picosecond-resolved optical spectroscopy. Formation of various sizes of QDs is observed by modifying the conditions suitably. The QDs also show tunable luminescence over the 680-800 nm spectral regions, with a quantum yield of 4-5%. The as-prepared QDs can serve as selective sensor materials for Hg(II) and Cu(II), based on selective luminescence quenching. The quenching mechanism is found to be based on Dexter energy transfer and photoinduced electron transfer for Hg(II) and Cu(II), respectively. The simple synthesis route of protein-capped HgS QDs would provide additional impetus to explore applications for these materials.

Engineering gold-based radiosensitizers for cancer radiotherapy

Radiotherapy remains a major modality in cancer therapy. The main goal of radiotherapy is to shrink tumors and kill cancer cells by using high-energy radiation, such as X-rays and γ-rays. Despite the effectiveness of radiotherapy in treating cancer in many possible ways, harmful damage caused by such radiation to the surrounding healthy/normal cells is often unavoidable. Therefore, it would be desirable to strike the right balance between tumor eradication and minimizing the possible side effects. To address this challenge, a magic bullet is the “radiosensitizer”, which can make cancer cells more sensitive to radiation by increasing the local treatment efficiency using a relatively low and safe radiation dose. This review article focuses on the recent progress in the area of radiosensitizers, especially gold-based materials. It begins with the key factors associated with radio-biological interactions along with the possible biological effects upon irradiation and from which the idea of radiosensitizers is derived. The prospect of gold-based materials as radiosensitizers is summarized through the mechanistic elucidation of the interaction of radiation with gold. Following that, some of the key design strategies for generating ideal gold-based radiosensitizers are highlighted. Examples of gold-based materials derived from these strategies, such as gold nanoparticles and nanoclusters, and their applications in radiotherapy are presented and discussed. The final part of this review article focuses on the avenues for future research on engineering gold-based nanomaterials for cancer radiotherapy.

Toward an Alternative Intrinsic Probe for Spectroscopic Characterization of a Protein

The intrinsic fluorescent amino acid tryptophan is the unanimous choice for the spectroscopic investigation of proteins. However, several complicacies in the interpretation of tryptophan fluorescence in a protein are inevitable and an alternative intrinsic protein probe is a longstanding demand. In this contribution, we report an electron-transfer reaction in a human transporter protein (HSA) cavity which causes the tryptophan residue (Trp214) to undergo chemical modification to form one of its metabolites kynurenine (Kyn214). Structural integrity upon modification of the native protein is confirmed by dynamic light scattering (DLS) as well as near and far circular dichroism (CD) spectroscopy. Femtosecond-resolved fluorescence transients of the modified protein describe the dynamics of solvent molecules in the protein cavity in both the native and denatured states. In order to establish general use of the probe, we have studied the dipolar interaction of Kyn214 with a surface-bound ligand (crystal violet, CV) of the protein. By using the sensitivity of FRET, we have determined the distance between Kyn214 (donor) and CV (acceptor). Our study is an attempt to explore an alternative intrinsic fluorescence probe for the spectroscopic investigation of a protein. In order to establish the efficacy of the modification technique we have converted the tryptophan residues of other proteins (bovine serum albumin, chymotrypsin and subtilisin Carlsberg) to kynurenine and confirmed their structural integrity. We have also shown that catalytic activity of the enzymes remains intact upon the modification.

Enhancing stability through ligand-shell engineering: A case study with Au25(SR)18 nanoclusters

While thiolate-protected Au nanoclusters (NCs) have drawn considerable interest in various fields, their poor stability in aqueous solution remains a major hurdle for practical applications. Here, we report a unique strategy based on ligand-shell engineering to improve the stability of thiolated Au NCs in solution. By employing two thiol-terminated ligands having oppositely charged functional groups on the surface of the NCs, we demonstrate that the electrostatic attraction between the oppositely charged functional groups of neighboring ligands could amplify the coordination among surface ligands, leading to the formation of pseudo-cage-like structures on the NC surface that could offer higher protection to the Au core in aqueous solution. The strategy developed in this study could be extended to other metal NCs, further paving the way toward practical applications.

MoS2 Nanocrystals Confined in a DNA Matrix Exhibiting Energy Transfer

We report the wet chemical synthesis of MoS2 nanocrystals (NCs), a transition-metal dichalcogenide, using DNA as a host matrix. As evidenced from transmission electron microscopy (TEM), the NCs are highly crystalline, with an average diameter of ~5 nm. Ultraviolet-visible (UV-vis) absorption studies along with band gap calculations confirm that NCs are in quantum confinement. A prominent red shift of the optical absorption bands has been observed upon formation of the thin film using hexadecyltrimethylammonium chloride (CTAC), i.e., in the case of MoS2@DNA-CTAC. In the thin film, strong electron-phonon coupling arises because of the resonance effect, which is reflected from the emergence of intense first-, second-, and third-order Raman peaks, whenever excited with the 488 nm line. We have established that our as-synthesized MoS2 NCs quench the fluorescence of a well-known DNA minor groove binding probe, Hoechst 33258. Unprecedented fluorescence quenching (94%) of donor (Hoechst 33258) emission and efficient energy transfer (89%) between Hoechst 33258 and MoS2 NCs (acceptor) are obtained. The donor-acceptor distance of these conjugates has been described by a Förster resonance energy transfer (FRET)-based model. Furthermore, employing a statistical method, we have estimated the probability of the distance distribution between the donor and acceptor. We believe that the study described herein may enable substantial advances in fields of optoelectronics, photovoltaics, catalysis, and many others.

Probing the Microporous Structure of Silica Shell Via Aggregation-Induced Emission in Au(I)-Thiolate@SiO2 Nanoparticle

An efficient method to investigate the window size of the silica shell generated via the classical Stöber method is reported by making use of the unique aggregation-induced emission property of Au(I)-thiolate complexes, which can precisely probe the porosity of the silica shell in Au(I)-thiolate@SiO2 nanoparticles.