Antibiotic Conjugated Fluorescent Carbon Dots as a Theranostic Agent for Controlled Drug Release, Bioimaging, and Enhanced Antimicrobial Activity

A novel report on microwave assisted synthesis of bright carbon dots (C-dots) using gum arabic (GA) and its use as molecular vehicle to ferry ciprofloxacin hydrochloride, a broad spectrum antibiotic, is reported in the present work. Density gradient centrifugation (DGC) was used to separate different types of C-dots. After careful analysis of the fractions obtained after centrifugation, ciprofloxacin was attached to synthesize ciprofloxacin conjugated with C-dots (Cipro@C-dots conjugate). Release of ciprofloxacin was found to be extremely regulated under physiological conditions. Cipro@C-dots were found to be biocompatible on Vero cells as compared to free ciprofloxacin (1.2？mM) even at very high concentrations. Bare C-dots (～13？mg？mL？1) were used for microbial imaging of the simplest eukaryotic model—Saccharomyces cerevisiae (yeast). Bright green fluorescent was obtained when live imaging was performed to view yeast cells under fluorescent microscope suggesting C-dots incorporation inside the cells. Cipro@C-dots conjugate also showed enhanced antimicrobial activity against both model gram positive and gram negative microorganisms. Thus, the Cipro@C-dots conjugate paves not only a way for bioimaging but also an efficient new nanocarrier for controlled drug release with high antimicrobial activity, thereby serving potential tool for theranostics. 1. Introduction Carbon quantum dots or carbon dots (C-dots) have become a colossal designation in the field of material science, since its serendipitous inception in 2004 during separation of multiwalled carbon nanotubes under electrical influence [1]. In medicine and theranostics, C-dots have emerged as new advancement owing to their exceptional biocompatibility [2], typical optical properties [3], nontoxic precursors as carbon sources, high aqueous solubility, and easy surface functionalization, unlike semiconductor quantum dots such as CdTe and CdSe. [4, 5]. Another catchy attribute of C-dots is their photoluminescence (PL) in near-infrared region (NIR) which can be potentially used for photothermal therapy of tumors [6, 7]. There is significant advancement in synthetic protocols for fabrication of fluorescent C-dots over the past few years. Most celebrated among them is microwave mediated synthesis [1], laser ablation of graphite [8], thermal cracking of organic compounds [9], electrooxidation of graphite [10], and oxidation of candle soot [11]. Moreover, there are very few reports on fabrication of C-dots using natural plant materials as carbon source. Recently, C-dot was synthesized using orange juice