Fluorescent carbon nanoparticle (CNP) having 2-6 nm in size with quantum yield of about ~3% were synthesized via nitric acid oxidation of carbon soot and this approach can be used for milligram scale synthesis of these water soluble particles. These CNPs are nano-crystalline with predominantly graphitic structure and shows green fluorescence under UV exposure. While nitric acid oxidation induces nitrogen and oxygen incorporation into soot particle that afforded water solubility and light emitting property; the isolation of small particles from a mixture of different size particles improved the fluorescence quantum yield. These CNP shows encouraging cell imaging application. They enter into cell without any further functionalization and fluorescence property of these particles can be used for fluorescence based cell imaging application.

Fluorescent nanoparticle-based imaging probes have advanced current labelling technology and are expected to generate new medical diagnostic tools based on their superior brightness and photostability compared with conventional molecular probes. Although significant progress has been made in fluorescent semiconductor nanocrystal-based biological labelling and imaging, the presence of heavy metals and the toxicity issues associated with heavy metals have severely limited the application potential of these nanocrystals. Here, we report a fluorescent carbon nanoparticle-based, alternative, nontoxic imaging probe that is suitable for biological staining and diagnostics. We have developed a chemical method to synthesise highly fluorescent carbon nanoparticles 1–10 nm in size; these particles exhibit size-dependent, tunable visible emission. These carbon nanoparticles have been transformed into various functionalised nanoprobes with hydrodynamic diameters of 5–15 nm and have been used as cell imaging probes.

Nanoparticle-based probes are emerging as alternatives to molecular probes with several advantages. A variety of water-soluble functional nanoparticles and nanobioconjugates need to be prepared and tested for this research. Development of appropriate coating chemistries is the key in deriving such functional nanoparticles. Herein we summarize different coating approaches those we have developed and compared them in the context of currently available coating methods, for the synthesis of soluble functional nanoparticles. We have focused on conventional ligand exchange, interdigited bilayer strategy, silica coating, polyacrylate coating, and imidazole based polymer coating and found that cross-linked coating, specifically by polyacrylate, provides a superior colloidal stability of nanoparticles. The robust coating provides the opportunity to explore various conjugation chemistries involving nanoparticle and to derive different soluble nanobioconjugates. A library of functional nanoprobes with hydrodynamic diameters of 10−100 nm are synthesized with these coating approaches which are composed of different nanoparticles (e.g., metal, metal oxide, or semiconductor) and affinity molecules (e.g., oligonucleotide, peptide, antibody, vitamins, etc.) and can be explored for cellular and subcellular imaging and ultrasensitive biosensing applications.

Although nanoparticle multivalency is known to influence their biological labeling performance, the functional role of multivalency is largely unexplored. Here we show that the folate receptor mediated cellular internalization mechanism of 35−50 nm nanoparticle shifts from caveolae-to clathrin-mediated endocytosis as the nanoparticle multivalency increases from 10 to 40 and results in the difference of their subcellular trafficking. We have synthesized folate functionalized multivalent quantum dot (QD) with varied average numbers of folate per QD between 10 and 110 [e.g., QD(folate) 10 , QD(folate) 20 , QD(folate) 40 , QD(folate) 110 ] and investigated their uptake and localization into folate receptor overexpressed HeLa and KB cells. We found that uptake of QD(folate) 10 occurs predominantly via caveolaemediated endocytosis and entirely trafficked to the perinuclear region. In contrast, uptake of QD(folate) 20 occurs via both caveolae-and chathrin-mediated endocytosis; uptake of QD(folate) 40 and QD(folate) 110 occurs predominantly via clathrin-mediated endocytosis and these three QDs localize predominantly at lysosome with restricted trafficking to the perinuclear region. This work shows the functional role of multivalent interaction in cellular endocytosis and intracellular trafficking which can be exploited for subcellular targeting applications.

Plasmonic-fluorescent composite nanoparticles are considered as unique, multifunctional nanoprobes for plasmon-and fluorescence-based imaging and detection. However, their synthesis is challenging due to fluorescence quenching of the fluorophore by plasmonic particles and most of the successful methods produce composite particles of large size (diameter > 50 nm), which limit their wider applications. Here we report 20-30 nm diameter plasmonic-fluorescent composite nanoparticles with reasonable fluorescence quantum yield (12-16%). These particles are composed of 3-6 nm diameter Au/Ag cores and fluorescein-incorporated polymeric shells. They have high water solubility, good colloidal stability, stable fluorescence properties, and are amenable in deriving various functional nanoprobes. Different functional nanoprobes are derived from these composites and successfully used for fluorescence-based cell labeling as well as plasmon-based detection applications.