Fluorescent carbon dot has emerged as promising alternative of conventionally known quantum dot or molecular probe as potential intracellular imaging probe. In particular, <10 nm size, tunable and bright fluorescence of carbon dot deserve the application potential as intracellular imaging probe. However, synthesis of carbon dot with narrow particle size distribution, preparation of high‐quality red/near‐infrared emitting carbon dot and appropriate design of functional carbon dot for subcellular targeting are most critical issues. This advanced review focus on the application potential of fluorescent carbon dot as intracellular imaging probe. At first, we briefly discuss different types of fluorescent carbon dots and origin of their fluorescence. Next, we focus on surface chemistry and functionalization which are relevant to intracellular probe development. Finally we have summarized various types of intracellular nanoprobes that are developed from fluorescence carbon dot. This article is categorized under: Diagnostic Tools > in vitro Nanoparticle‐Based Sensing

Fluorescent carbon nanoparticle-based probes with tunable visible emission are biocompatible, environment friendly and most suitable for various biomedical applications. However, synthesis of red fluorescent carbon nanoparticles and their transformation into functional nanoparticles are very challenging. Here we report red fluorescent carbon nanoparticle-based nanobioconjugates of <25 nm hydrodynamic size and their application as fluorescent cell labels. Hydrophobic carbon nanoparticles are synthesized via high temperature colloid-chemical approach and transformed into water-soluble functional nanoparticles via coating with amphiphilic polymer followed by covalent linking with desired biomolecules. Following this approach, carbon nanoparticles are functionalized with polyethylene glycol, primary amine, glucose, arginine, histidine, biotin and folic acid. These functional nanoparticles can be excited with blue/green light (i.e., 400-550 nm) to capture their emission spanning from 550 to 750 nm. Arginine and folic acid functionalized nanoparticles have been demonstrated as fluorescent cell labels where blue and green excitation has been used for imaging of labeled cells. The presented method can be extended for the development of carbon nanoparticle-based other bioimaging probes.

Although nanoparticle-tagged antimicrobal peptides have gained considerable importance in recent years, their structure-function correlation has not yet been explored. Here, we have studied the mechanism of action of a designed antimicrobial peptide, VG16KRKP (VARGWKRKCPLFGKGG), delivered via gold nanoparticle tagging against Salmonella infection by combining biological experiments with high- and low-resolution spectroscopic techniques. In comparison with the free VG16KRKP peptide or gold nanoparticle alone, the conjugated variant, Au-VG16KRKP, is non-cytotoxic to eukaryotic cells, but exhibits strong bacteriolytic activity in culture. Au-VG16KRKP can penetrate host epithelial and macrophage cells as well as interact with intracellular S. Typhi LPS under both in vitro and in vivo conditions. Treatment of mice with Au-VG16KRKP post-infection with S. Typhi resulted in reduced intracellular bacterial recovery and highly enhanced protection against S. Typhi challenge. The three-dimensional high resolution structure of nanoparticle conjugated VG16KRKP depicted the generation of a well-separated amphipathic structure with slight aggregation, responsible for the increase of the local concentration of the peptide, thus leading to potent activity. This is the first report on the structural and functional characterization of a nanoparticle conjugated synthetic antimicrobial peptide that can kill intracellular pathogens and eventually protect against S. Typhi challenge in vivo.

Although fluorescent carbon nanoparticles have enormous biomedical application potential, the synthesis of high quality red fluorescent carbon nanoparticles is challenging. Here we report water dispersible, red fluorescent carbon nanoparticles of 10–25 nm hydrodynamic size with the fluorescence quantum yield of 25%. The approach involves controlled carbonization of resorcinol in ethylene glycol–Na3PO4 at 190 °C under air exposure. At this condition resorcinol undergoes oxidative phenol coupling associated with dehydration that leads to the nucleation of a π-conjugated 2D graphitic sheet followed by growth into a condensed graphitic core. The method is used to prepare 50–60 mg of particles in one batch and can be easily adapted for gram scale synthesis. The nanoparticles maintain good colloidal stability and fluorescence stability under physiological conditions, display concentration dependent and reversible transition between red to green fluorescence, and can be conjugated with primary amine terminated material by simple incubation. This nanoparticle can be used as a bioimaging probe via conversion into different colloidal nanobioconjugates.

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