We demonstrate here the effective delivery of a dye payload into cells using 2-nm core gold nanoparticles, with release occurring via place exchange of glutathione onto the particle surface. In vitro experiments demonstrate effective release of drug analogues upon addition of glutathione. Cell culture experiments show rapid uptake of nanoparticle and effective release of payload. The role of glutathione in the release process was demonstrated through improved payload release upon transient increase in glutathione levels achieved via introduction of glutathione ethyl ester into the cell.

Thioalkyl and thioalkylated oligo(ethylene glycol) (OEG) ligands with chain-end functionality were used to fabricate water-soluble CdSe nanoparticle scaffolds. Surface recognition of chymotrypsin (ChT) was achieved using these functionalized nanoparticle scaffolds, with three levels of interaction demonstrated: no interaction (OEG terminated with hydroxyl group), inhibition with denaturation (carboxylate-terminated thioalkyl ligands), and inhibition with retention of structure (carboxylate-terminated OEG). The latter process was reversible upon an increase in ionic strength, with essentially complete restoration of enzymatic activity.

FePt magnetic nanoparticles (MNPs) were functionalized with a mixed monolayer of poly(ethylene glycol)-terminated thiol and dopamine ligands. The resulting nanoparticles were soluble and stable in aqueous media, including water, ionic solutions, and cell culture medium. The surface thiol ligands are readily exchanged with other thiols bearing chain-end functionalities. MNPs featuring either a cationic or an anionic surface were synthesized by ligand exchange chemistry to afford ligand peripheries capable of binding biomolecules. Surface binding of cationic MNPs to DNA and anionic MNPs to chymotrypsin was enabled by incorporation of a charged functionality on the nanoparticle surface. This approach represents a general strategy to synthesize functionalized FePt nanoparticles that form stable solutions in water and facilitates the use of these magnetic FePt nanoparticles in biological applications.

We demonstrate the patterning of silica substrates with thymine (Thy-PS) and positively charged N-methylpyridinium (PVMP) polymers using photolithography and the subsequent orthogonal modification of these surfaces using diaminopyridine-functionalized polystyrene (DAP-PS) and carboxylate-derivatized CdSe/ZnS core-shell nanoparticles (COO-NP) through diamidopyridine-thymine three-point hydrogen bonding and pyridinium-carboxylate electrostatic interactions, respectively. This two-component orthogonal surface modification was accomplished in a self-sorting, single-step fashion, providing a versatile tool for the rapid and efficient creation of complex materials.

We report here on a new amphiphilic homopolymer that binds noncovalently to proteins. This polymer not only binds to the target protein chymotrypsin with submicromolar affinity but also stabilizes the native structure of the protein. Since the polymer-protein binding process is based on electrostatic interaction, the bound protein can be released from the polymer surface and reactivated either by increasing the ionic strength or by adding complementary cationic surfactants. The electrostatic binding of polymer to the protein results in a marked change in the substrate specificity of chymotrypsin.

Electrostatic interactions were used to noncovalently conjugate chymotrypsin to gold nanoparticles featuring hybrid tetraethylene(glycol)alkanethiol monolayers terminated with carboxylate groups. This conjugation process greatly alters the substrate selectivity of the adsorbed chymotrypsin, inhibiting the hydrolysis of anionic subtrates without affecting the hydrolysis rate of cationic analogues.

Positively charged trimethylammonium-functionalized mixed monolayer protected clusters (MMPCs) bind DNA through complementary electrostatic interactions, resulting in complete inhibition of DNA transcription of T7 RNA polymerase. DNA was released from the nanoparticle by intracellular concentrations of glutathione, resulting in efficient transcription. The restoration of RNA production was dose-dependent in terms of GSH, with considerable control of the release process possible through variation in monolayer structure. This work presents a new approach to controlled release of DNA, with potential applications in the creation of transfection vectors and gene regulation systems.

Magnetic particles can act as magnetic relaxation switches (MRSw's) when they bind to target analytes, and switch between their dispersed and aggregated states resulting in changes in the spinspin relaxation time (T 2 ) of their surrounding water protons. Both nanoparticles (NPs, 10-100 nm) and micron-sized particles (MPs) have been employed as MRSw's, to sense drugs, metabolites, oligonucleotides, proteins, bacteria and mammalian cells. To better understand how NPs or MPs interact with targets, we employed as a molecular recognition system the reaction between the Tag peptide of the influenza virus hemagglutinin and a monoclonal antibody to that peptide (anti-Tag). To obtain targets of different size and valency, we attached the Tag peptide to BSA (Mw= 65000 Daltons, diameter = 8 nm) and to Latex spheres (diameter = 900 nm). To obtain magnetic probes of very different sizes, anti-Tag was conjugated to 40 nm NPs and 1 μm MPs. MP and NP probes reacted with Tag peptide targets in a manner similar to antibody/antigen reactions in solution, exhibiting socalled prozone effects. MPs detected all types of targets with higher sensitivity than NPs with targets of higher valency being better detected than those of lower valency. The Tag/anti-tag recognition system can be used to synthesize combinations of molecular targets and magnetic probes, to more fully understand the aggregation reaction that occurs when probes bind targets in solution and the ensuing changes in water relaxation times that result.