Bacterial Synthesis of Ternary CdSAg Quantum Dots through Cation Exchange: Tuning the Composition and Properties of Biological Nanoparticles for Bioimaging and Photovoltaic Applications.

Nicolás Órdenes-Aenishanslins,Paras N Prasad,Alejandra Tello,Denisse Bravo,Daniela Toro-Ascuy,Giovanna Anziani-Ostuni,Juan Pablo Monrás,Ricardo Soto-Rifo,José Manuel Pérez-Donoso

doi:10.3390/microorganisms8050631

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.

Highlights

Synthesis of nanoparticles using microorganisms represents a novel alternative to traditional chemical synthesis that allows economical production of biocompatible nanostructures with a tunable size and physical properties
Bacterial synthesis of Quantum Dots (QDs) can be determined by detecting fluorescence emission in cell pellets and supernatants [11,14]
E. coli cells at an exponential growth phase were exposed to Cd+2 and cysteine, and the time-dependent shift in NPs growth was determined by measuring QDs-associated bacterial fluorescence in cell cultures exposed to UV light

Summary

Introduction

Synthesis of nanoparticles using microorganisms (biosynthesis) represents a novel alternative to traditional chemical synthesis that allows economical production of biocompatible nanostructures with a tunable size and physical properties. Different biomolecules interact with these in-situ produced nanoparticles (NPs) by modifying their surface and improving their stability as well as their biological function and cytotoxicity [1,2]. QDs biosynthesis is a more complex process than biological synthesis of metal nanoparticles (e.g., Cu0, Te0, Ag0) because the formation of nanostructures with multiple compositions requires the interaction of elements in a certain oxidation state (e.g., Cd2+, S2−, Te2−, Ag+). E. coli does not have these systems, H2S is generated in the presence of L-cysteine as a byproduct of cysteine desulfhydrase enzymatic activity [16] In this sense, the use of L-cysteine as a H2S precursor in E. coli cells and the use of a purified cysteine desulfhydrase enzyme for the biological synthesis of CdS nanoparticles have been recently reported [17,18]

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Results

Conclusion