No abstract

In this study, we introduce a biological method for the production of ternary Quantum Dots (QDs): complex nanostructures with tunable optical and structural properties that utilizes post-synthesis modifications through cation exchange. This versatile in-situ cation exchange method being reported for the first time shows great potential for extending the scope of microbial synthesis. By using this bacterial-based method, we easily synthesize and purify CdS, CdSAg, and Ag2S nanocrystals of a size below 15 nm and with variable morphologies that exhibit fluorescence emissions covering a broad spectral range (from 400 to 800 nm). Energy-dispersive X-ray spectroscopy (EDS) results indicate the partial replacement of Cd2+ by Ag+ when AgNO3 concentration is increased. This replacement produces CdSAg ternary QDs hetero-structures with high stability, fluorescence in the NIR-I (700 - 800 nm), and 36.13% quantum yield. Furthermore, this reaction can be extended for the production of soluble Ag2S nanoparticles (NPs) without any traces of Cd. QDs biosynthesized through this cation exchange process display very low toxicity when tested in bacterial or human cell lines. Biosynthesized ternary hetero-structures were used as red fluorescent dyes to label HeLa cells in confocal microscopy studies, which validates its use in bioimaging applications in the near infrared region. In addition, the application of biologically-produced cadmium NPs in solar cells is reported for the first time. The three biosynthesized QDs were successfully used as photosensitizers, where the CdSAg QDs show the best photovoltaic parameters. Altogether, obtained results validate the use of bacterial cells for the controlled production of nanomaterials with properties that allow their application in diverse technologies. We developed a simple biological process for obtaining tunable Quantum Dots (QDs) with different metal compositions through a cation exchange process. Nanoparticles (NPs) are produced in the extracellular space of bacterial cells exposed to cysteine and CdCl2 in a reaction that depends on S2− generation mediated by cysteine desulfhydrase enzymes and uses cellular biomolecules to stabilize the nanoparticle. Using this extracellular approach, water-soluble fluorescent CdS, CdSAg, and Ag2S Quantum Dots with a tunable emission ranging from 400 to 800 nm were generated. This is the first study reporting the use of microorganisms to produce tunable ternary QDs and the first time that a cation exchange process mediated by cells is described. Obtained results validate the use of biological synthesis to produce NPs with new characteristics and opens a completely new research field related to the use of microorganisms to synthesize complex NPs that are difficult to obtain with regular chemical methods.

In the present work, we report the use of bacterial cells for the production of CdS/CdSe Core/Shell quantum dots (QDs), a complex nanostructure specially designed to improve their performance as photosensitizer in photovoltaic devices. The method requires the incorporation of L-cysteine, CdCl 2 and Na 2 SeO 3 to Escherichia coli cultures and allows a tight control of QDs properties. The obtained CdS/CdSe QDs were photophysically and structurally characterized. When compared to CdS QDs, the classical shift in the UV-visible spectra of Core/Shell nanostructures was observed in CdS/CdSe QDs. The nanosize, structure, and composition of Core/Shell QDs were confirmed by TEM and EDS analysis. QDs presented a size of approximately 12 nm (CdS) and 17 nm (CdS/CdSe) as determined by dynamic light scattering (DLS), whereas the fourier transform infrared (FTIR) spectra allowed to distinguish the presence of different biomolecules bound to both types of nanoparticles. An increased photostability was observed in CdS/CdSe nanoparticles when compared to CdS QDs. Finally, biosynthesized CdS/CdSe Core/Shell QDs were used as photosensitizers for quantum dots sensitized solar cells (QDSSCs) and their photovoltaic parameters determined. As expected, the efficiency of solar cells sensitized with biological CdS/CdSe QDs increased almost 2.5 times when compared to cells sensitized with CdS QDs. This work is the first report of biological synthesis of CdS/CdSe Core/Shell QDs using bacterial cells and represents a significant contribution to the development of green and low-cost photovoltaic technologies.

Preparation of concentrated silver nanoparticles in water remains a challenge today. The intrinsic reactivity of silver, as well as the high surface energy of nanoparticles, make it difficult to handle them without altering their pristine properties. Herein, we report the preparation of concentrated silver nanoparticles (AgNPs) dispersion (2 mM; 1.5⋅10 15 NPs/mL) by reducing Ag + in-situ of a β-cyclodextrin-epichlorohydrin polymer (βCDP) as a capping agent. The prepared nanoparticles (AgNPs@βCDP) with a Surface Plasmon Resonance band at 396 nm, and a hydrodynamic diameter of 21.4 � 1.8 nm, retained both features after being precipitated and redispersed in water. The AgNPs core had a spherical morphology, with a 12.7 � 1.5 nm diameter in size, as determined by TEM. The AgNPs@βCDP showed outstanding bactericidal properties against Escherichia coli (MIC = 0.37 nM), one of the lowest ever achieved for silver nanoparticles. We suggest that the polymer acts as a Trojan horse with AgNPs as payload.

A one-pot green method for aqueous synthesis of fluorescent copper sulphide nanoparticles (NPs) was developed. The reaction was carried out in borax–citrate buffer at physiological pH, 37 °C, aerobic conditions and using Cu (II) and the biological thiol cysteine. NPs exhibit green fluorescence with a peak at 520 nm when excited at 410 nm and an absorbance peak at 410 nm. A size between 8–12 nm was determined by dynamic light scattering and transmission electron microscopy. An interplanar atomic distance of (3.5 ± 0.1) Å and a hexagonal chalcocite crystalline structure (βCh) of Cu2S NPs were also determined (HR-TEM). Furthermore, FTIR analyses revealed a Cu-S bond and the presence of organic molecules on NPs. Regarding toxicity, fluorescent Cu2S NPs display high biocompatibility when tested in cell lines and bacterial strains. Electrocatalytic activity of Cu2S NPs as counter electrodes was evaluated, and the best value of charge transfer resistance (Rct) was obtained with FTO/Cu2S (four layers). Consequently, the performance of biomimetic Cu2S NPs as counter electrodes in photovoltaic devices constructed using different sensitizers (ruthenium dye or CdTe NPs) and electrolytes (S2−/Sn2− or I−/I3−) was successfully checked. Altogether, novel characteristics of copper sulfide NPs such as green, simple, and inexpensive production, spectroscopic properties, high biocompatibility, and particularly their electrochemical performance, validate its use in different biotechnological applications.