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.

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.

Multivalency of nanoparticle and associated cooperative binding with biological interface is an important aspect in the development of nanoparticle based bioimaging probes. However, the preparation of such a nanobioconjugate with a controlled number of biomolecules per nanoparticle, typically between 1 and 100, is challenging. Here we report a generalized two-step bioconjugation method to prepare nanobioconjugates with a varied average number of biomolecules between 1 to 100 per nanoparticle that can be applied to different nanoparticles and biomolecules. Following this approach we have successfully synthesized quantum dot (QD) based bioconjugates with controlled average numbers of glucose or folate and found their number-dependent interaction with proteins and cells. We propose a method for exploiting the nanoparticle multivalency effect toward various biological interactions and preparing such nanobioconjugates for best performance.

Water soluble graphene with various chemical- and biofunctionalities is essential for their different applications. However, exfoliated graphenes are insoluble in water and water soluble graphene oxide precipitate if they are chemically reduced to graphene. We have developed a polyacrylate coating method for graphene oxide and then chemically reduced it into graphene. We found that polyacrylate coating can improve the colloidal stability of both graphene and graphene oxide. The coated graphene has been characterized using XPS, FTIR, XRD and micro-Raman spectroscopy. The primary amine present on the coating backbone has been used to derive glucose functionalized water soluble graphene. Various other functional graphenes can be anticipated from the polyacrylate coated graphene.

Functionalization of gold nanorods is a key issue for their biomedical application, and currently it is performed via either electrostatic interaction or thiol based strategy. We have developed a polyacrylate based coating chemistry for gold nanorods that can be used in deriving a variety of functional nanorods with high colloidal stability. The coating processes can introduce primary amines, fluorescein, or poly(ethylene glycol) (PEG) on the nanorod surface in one step process. While fluorescein incorporation can produce fluorescent nanorods, primary amine groups can be used for further functionalization. Various functional nanorods have been successfully synthesized from these coated nanorods and used in different applications. Glucose and biotin functionalized nanorods are used for protein detection, and oleyl functionalized nanorods with fluorescein incorporated in the polymer shell are used for fluorescence based cell labeling.