Recently, medical research has been shifting its focus to nanomedicine and nanotherapeutics in the pursuit of drug development research. Quantum dots (QDs) are a critical class of nanomaterials due to their unique properties, which include optical, electronic, and engineered biocompatibility in physiological environments. These properties have made QDs an attractive biomedical resource such that they have found application as both in vitro labeling and in vivo theranostic (therapydiagnostic) agents. Considerable research has been conducted exploring the suitability of QDs in theranostic applications, but the cytotoxicity of QDs remains an obstacle. Several types of QDs have been investigated over the past decades, which may be suitable for use in biomedical applications if the barrier of cytotoxicity can be resolved. This review attempts to report and analyze the cytotoxicity of the major QDs along with relevant related aspects.

Gold nanoclusters are promising candidates as biological markers without having toxic effects like fluorescent quantum dots. Herein, bovine serum albumin (BSA) protein stabilized gold nanoclusters of two different sizes emitting at 410 and 645 nm have been synthesized. These nanoclusters have been shown to interact with molecular oxygen differentially. Spectroscopic and chemical evidences show that dioxygen molecule gets adsorbed at two different orientations on the nanoclusters. The orientation motifs have been hypothesized to be superoxo and peroxo types on the smaller and the larger gold nanoclusters, respectively. Due to the difference in attachments, the oxygen molecule shows opposite changes in fluorescence intensity for the nanoclusters. The fluorescence intensity of the blue emitting nanocluster shows a profuse enhancement whereas the red emitting species shows quenching of emission. Superoxo type adsorption of the oxygen molecule on the blue emitting gold nanoclusters induce formation of singlet oxygen that in turn enhances the fluorescence intensity of the species. This could be verified by oxidation of diaminobenzidine (DAB) by singlet oxygen. Enhancement in fluorescence intensity of the blue emitting gold nanoclusters with an increase in concentration of molecular oxygen may enable them to be good candidates in bioimaging and detection.

Nanotechnology has emerged as one of the leading research areas involving nanoscale manipulation of atoms and molecules. During the past decade, the growth of nanotechnology has been one of the most important developments that have taken place in the biomedical field. The new generation nanomaterials like Quantum dots are gaining much importance. Also, there is a growing interest in the development of nano-theranostics platforms in medical diagnostics, biomedical imaging, drug delivery, etc. Quantum dots are also known as nanoscale semiconductor crystals, with unique electronic and optical properties. Recently, silicon quantum dots are being studied extensively due to their less-toxic, inert nature and ease of surface modification. The silicon quantum dots (2-10nm) are comparatively stable, having optical properties of silicon nanocrystals. This review focuses on silicon quantum dots and their various biomedical applications like drug delivery regenerative medicine and tissue engineering. Also, the processes involved in their modification for various biomedical applications along with future aspects are discussed.

No abstract