Luminescent Carbon Dots: An Emerging Nanomedicine to Treat Antibiotic Resistant Bacterial Infections

Globally, the number of antibiotic resistant bacterial infections is increasing significantly, while conventional antibiotics, recommended as a gold standard medicine have now become futile. Therefore, demand for an alternative of antibiotics is on for a long time. In this regard, the advent of nanoscience and technology offers huge possibilities to develop a new class of manmade tiny particles in the range of nanometer dimensions. Nanomedicine,application of the nanoscale materials in the field of medicine, provides several innovative solutions for im-provement of healthcare system by offering novel nano-therapeutics and diagnostic tools with better performance and accuracy.

The field of nanomedicine is associated with various allied areas of sciences, which are not mere extension of molecular medicine.These arecorrelated with the possibilities of nanomedicines offering a great deal of scope for generation of novel theranostic agents that would provide ‘see and treat’ approach effective for simultaneous diagnosis and treatment. This allows manipulation of materials with atomic precision to produce nanostructures with size comparable tothe biomolecules for specific and targeted interactions with cells and /or organelles. The precise architecture and unique properties of these nanoscale materials allow them to lance through biological barriers including bacterial cell walls. One such nanoscale material is carbon dots (C-dots)—quasisphericalnanoparticles having a size less than 10 nm, mainly comprised of carbon as a core element. However, depending on the method of synthesis, traces of oxygen and nitrogen may also present. Moreover, various types of heteroatoms such as N, S, P, and B can be doped resulting in an alteration of the optical response of C-dots.

The C-dotsare generally biocompatible in nature, hence easily degrade in the biological environment and less toxic to human health as compared to other nanomaterials. Interestingly scientists had reported that grilled and baked starch containing foods have brown parts on their outside which is nothing but amorphous C-dots. This implies that bread toast we consume every morning contains C-dots in its brown part. Likewise, dry heating of sugar develops dark brown color residues, which are nothing but crude forms of C-dots. Thus, chemists are taking this idea inside the laboratory in a better way to do the synthesis of C-dots on large scale by using economical precursors such as starch, glucose, etc. The recent advancement of the synthesis methods provide the scope of addition of various functional groups on its surface that increases its water solubility and stabilityapposite for the biological applications. Apart from this, currently, there are studies that demonstrated synthesis of C-dots from organic waste and plant biomass. This led to a reduction of the production cost as well as offered great scope of large-scale production based on renewable and highly abundant resources, particularly suitable for industrial production.

The C-dots possess remarkable optical properties. It shows strong luminescence from blue to near infra-red (NIR) depending on the size and surface chemistry. This emerging ‘nano-light’ has been considered as a new horizon of luminescent agents having high photostability, non-blinking, and tunable excitation properties. However, the origin of luminescence is not fully explored yet. Several hypothesis have been proposed including quantum confinement effect, exciton recombination radiation, or stabilising surface trap effects. The tunable luminescent properties of C-dots promise as new fluorescent probe for a wide range of applications. Researchers developed several chemical and biochemical sensors based on the luminescent C-dots. Besides, it shows huge potential to develop luminescent ink for various calligraphic and anti-counterfeiting applications, which are particularly ‘on-demand’ for various military and security purposes.

The C-dots act as both donors and acceptors of electrons based on the chemical reaction. Due to this unique electron transferability, it acts as a good catalyst for several chemical reactions. This also showed enormouspossibilities in various optoelectronics, and energy storage applications. It would be mentioned here that overwhelming positive vibes about C-dots are due to its multifarious applications and biologically acceptable properties, which also overcome the limitations of Quantum Dots (QDs) - that is also another class of nanoscale materials that gained vast popularity from the scientific community for last few decades. QDs are semiconductor nanocrystals usually made up with heavy metals, thus raising the concern of toxicity, which limits their further application in the field of nanomedicine.

Owing to these applications, C-dots might play a very essential role in the next generation medicines with the ability to kill both Gram positive and Gram negative bacteria. In an attempt to find out alternatives of antibiotics, C-dots are being tested as a competent antibacterial agent against resistant bacterial infection too. It would be worth mentioning here that improper and incomplete dosage of antibiotics have resulted in the emergence of antibiotic resistant bacterial infections worldwide, which are not curable by the conventional antibiotics available currently. As per Global Antimicrobial Surveillance System (GLASS) reports in 2018 ofthe World Health Organization, antibiotic resistance was found in over 0.5 million people with suspected bacterial infections across 22 countries. Statistics from different countries revealed that 0-82 per cent of patients with suspected bloodstream infection showed resistance to at least one of the majorly used antibiotics to treat the same. For instance, countries reported 0-51% penicillin resistance, an antibiotic used to treat pneumonia. Similarly 8-65% ciprofloxacin resistance was observed in patients being treated for urinary tract infections.

The number of death associated with these resistant bacterial infections is growing very rapidly and probably reach up to 10 million deaths per year by 2050, if left unattended. To understand the impact of the situation and take a competitive measure the World Health Organization (WHO) in 2017 published the first list of antibiotic-resistant bacteria. There are several bacterial species that cause huge concern e.g. methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), and drug-resistant Streptococcus pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterobacter spp. These resistant species are able to overcome the challenges of antibiotics by adopting one or more unconventional biochemical pathways such as alteration of the chemical structure of antimicrobial agents, increase the efflux of the antibiotics from the cell, modification of binding site of the antibiotics and degrading the antibiotics directly.

Therefore, these multidrug-resistant bacteria (MDR) and extensive drug-resistant bacteria (XDR) became a serious global threat despite the recent developmentsinmedical sciences and drug discovery. To address this grim situation, there is an urgent need for the development of alternatives that may be effective alone or in combination with antibiotics. In this regard use of nanoscale materials gain an edge over conventional therapeutics. It is a well-accepted fact that the use of nanoscale materials offers greater contact with bacteria owing to its size and tunable surface properties. This also has improved bioavailability and better half-life as compared to conventional drugs.

For last few decades, various metal and metal oxide nanoparticles were in limelight exhibiting significant ability as potential bactericidal agents against a wide range of Gram positive and Gram negative bacterial strains. However, in some cases leaching of metal ions raises concern of toxicity, which limits its direct use as nano-therapeutics. In this regard the use of C-dots-an organic nanoscale materials- has a potential bactericidal agent established by various researchers globally.Moreover, the biocompatibility and less toxicity inspires to explore these newly synthesized nanoscale materials in a better way as novel alternative antibacterial agents, which have less chances of generation of resistant bacterial strains.

The C-dots may induce multiple pathways simultaneously depending on their physicochemical properties. Recent studies demonstrated that size, surface functional groups, overall surface charge, and polarity decided bactericidal fate of the C-dots.  The dimension of C-dots plays a very essential role in its antibacterial activity as it offers direct ‘particle effects’-penetrates bacterial cell wall that leads to leak out of intracellular components. Apart from the size, our recent study showed that positive and neutral charged C-dots revealed higher bactericidal properties as compared to negatively charged C-dots. Likewise C-dots are also capable of producing reactive oxygen species (ROS) which leads to higher bactericidal activity of the C-dots. This line of interest also proposed that by using light energy, C-dots employ as photo-activated antimicrobial agents, which causes photo-induced redox processes for generation of ROS. Actually, C-dots undergo a rapid charge separation for the formation of electrons and holes similar to conventional QDs, which eventually confine at various surface sites by suitable surface passivating agents. It proposes that radiative recombination of the electrons and holes are one of the responsible causes for the luminescence of C-dots. The C-dots showed strong photodynamic effects that have been employed to eradicate bacterial cells by illumination of visible light. Moreover, the photo excited electrons from C-dots could also be used for single-electron reduction of the oxygen inside the bacterial cells which ultimately produces cascade of free radicals, commonly known as ROS. In combination with the antibiotics, C-dots showed synergistic bactericidal activity. For example, amine-terminated carbon dots functionalised with ampicillin (AMP) offer strong bactericidal activity as compared to its individual components. Our group also demonstrated that C-dots might be delivered with Ag NPs for combination therapy to kill both wild type Gram positive and Gram negative bacteria viz. Escherichia coli, Bacillus subtilis, Enterobacter aerogenes. The observed heightened bactericidal effects were assigned to a higher level of intracellular ROS productions due to electron transfer from C-dots to oxygen via Ag NPs. Interestingly, this combination therapy was showing a synergistic effect against ampicillin resistant bacterial strains.

The newly synthesized carbon rich nano-therapeutic coined as C-dots is found to be one of the most promising and potential bactericidal agents for the prevention and cure of drug-resistant bacterial infections. Further, studies showed that combination therapy of antibiotics with C-dots has the prospective to open a new avenue in order to kill drug-resistant bacterial infection.Thus, research groups around the world have been involved in developing a new synthesis method, which will be economical and offer the possibility for large-scale production of the biocompatible C-dots. Although significant research need to be performed in this line of interest for optimisation, standardisation and toxicity study, it is deemed that the emerging discipline of nanomedicine takes material sciences and medicine closer for development of novel therapeutics like fluorescent C-dots as an alternative of antibiotics-that will be indeed very effective value addition to the realm of medicines.