Although the research activities pertaining to the synthesis of fluorescent noble metal nanoclusters (NCs) and their applications in biological optics have been growing, only limited information is available in the near IR (NIR) region. However, fluorescence spectroscopy and microscopy in the NIR region offer significant advantages over UV and visible wavelengths. In this manuscript, we demonstrate biomineralized synthesis of stable Au-Ag bimetallic NCs with tunable NIR fluorescence using bovine serum albumin (BSA) as a protein template. We also demonstrate its application in the detection of toxic heavy metal ions Pb 2+ in vitro and inside cells. The tunability of the fluorescence emission between 680 nm and 815 nm is achieved by systematically varying the ratio of Au and Ag in the composite NCs. The bimetallic NCs when interacting with Pb 2+ offered a large increase in fluorescence intensity, which enabled sensitive detection of Pb 2+ . We determined a limit of detection (LOD) of 96 nM for the detection of Pb 2+ under in vitro conditions, which is significantly less than the safe level in drinking water. Its applicability has also been demonstrated successfully in real water samples collected from local water bodies. † Electronic supplementary information (ESI) available: Two additional tables describing the synthetic optimization in terms of Au/Ag molar ratio variations and time resolved study with respect to Au/Ag variation have been mentioned. Absorption spectra of (Au-Ag@BSA) opt NCs; MALDI mass spectrum of the nanoclusters; X-ray photoelectron spectra; uorescence stability of nanoclusters in the presence of EDTA, NaCl and different pH; stability of (Au-Ag@BSA) opt NCs with time; variation of nanocluster uorescence intensity with increasing Ag concentration. See

Biomimetic synthesis of multifunctional fluorescent gold nanoclusters (Au NCs) is of great demand because of their ever-increasing applications. In this study, we have used self-assembled bovine serum albumin (BSA) amyloid-like nanofibers as the bioinspired scaffold for the synthesis of Au NCs. The amyloid fibril stabilized gold nanocluster (Fib-Au NC) has been found to have appreciable enhancement of fluorescence emission and a large 25 nm red shift in its emission maxima when compared to its monomeric protein counterpart (BSA-Au NC). The underlying mechanism accountable for the fluorescence behavior and its spectral shift has been thoroughly investigated by a combined use of spectroscopic and microscopic techniques. We have subsequently demonstrated the use of Fib-Au NCs for cysteine (Cys) sensing both in vitro and inside live cells. Additionally, cellular uptake and postpermeation effect of Fib-Au NCs have also been ascertained by detailed flow cytometry analysis, viability assay, and real-time apoptotic gene expression profiling.

Virulent Mycobacterium tuberculosis (MTB) strains cause cell death of macrophages (Mϕ) inside TB granuloma using a mechanism which is not well understood. Many bacterial systems utilize toxins to induce host cell damage, which occurs along with immune evasion. These toxins often use chameleon sequences to generate an environmentsensitive conformational switch, facilitating the process of infection. The presence of toxins is not yet known for MTB. Here, we show that MTBsecreted immunogenic MPT63 protein undergoes a switch from β-sheet to helix in response to mutational and environmental stresses. MPT63 in its helical form creates pores in both synthetic and Mϕ membranes, while the native β-sheet protein remains inert toward membrane interactions. Using fluorescence correlation spectroscopy and atomic force microscopy, we show further that the helical form undergoes self-association to produce toxic oligomers of different morphology. Trypan blue and flow cytometry analyses reveal that the helical state can be utilized by MTB for killing Mϕ cells. Collectively, our study emphasizes for the first time a toxin-like behavior of MPT63 induced by an environment-dependent conformational switch, resulting in membrane pore formation by toxic oligomers and Mϕ cell death.