Nucleic-acid-functionalized CdSe/ZnS quantum dots (QDs) were hybridized with the complementary Texas-Red-functionalized nucleic acid. The hybridization was monitored by following the fluorescence resonance energy transfer from the QDs to the dye units. Treatment of the QD/dye DNA duplex structure with DNase I resulted in the cleavage of the DNA and the recovery of the fluorescence properties of the CdSe/ZnS QDs. The luminescence properties of the QDs were, however, only partially recovered due to the nonspecific adsorption of the dye onto the QDs. Similarly, nucleic-acid-functionalized Au nanoparticles (Au NPs) were hybridized with the complementary Texas-Red-labeled nucleic acid. The hybridization was followed by the fluorescence quenching of the dye by the Au NPs. Treatment of the Au NP/dye DNA duplex with DNase I resulted in the cleavage of the DNA and the partial recovery of the dye fluorescence. The incomplete recovery of the dye fluorescence originated from the nonspecific binding of the dye units to the Au NPs. The nonspecific binding of the dye to the CdSe/ZnS QDs and the Au NPs is attributed to nonprotected surface vacancies in the two systems.

CdSe/ZnS QDs enable the optical probing of the biocatalytic oxidation of tyrosine derivatives and of the scission of peptides by thrombin. CdSe/ZnS QDs were modified with tyrosine methyl ester or with a tyrosine-containing peptide. The tyrosine units were reacted with tyrosinase/O2 to yield the respective l-DOPA and quinone derivatives. The luminescence of QDs modified by the enzyme-generated quinone units is quenched. The quinone-functionalized peptide associated with the QDs was cleaved by thrombin, a process that restored the luminescence of the QDs.

Two different types of nanoparticles dissolved in organic solution, gold stabilized by n-alkanethiols and CdSe/ZnS stabilized by n-alkane-amine, adhere preferentially to the ridges of latent fingerprints; the gold deposits catalyze silver electroless deposition from "Silver Physical Developer" (Ag-PD), an aqueous solution containing silver colloids stabilized by cationic surfactants, to form dark impressions of the ridge details; the hydrophobic capped gold nanoparticles significantly improve the intensity and clarity of the developed prints compared with Ag-PD alone; finger marks treated with CdSe/ZnS nanoparticles can be viewed directly, due to their fluorescence under UV illumination.

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The use of semiconductor quantum dots (QDs) as optical labels for biorecognition events and biocatalytic processes attracts growing interest. [1,2] While numerous studies reported on the use of the QDs as fluorescent labels, [3][4][5][6] applications of semiconductor QDs as optical probes of dynamic bioprocesses, such as enzymatic transformations, using fluorescence resonance energy transfer (FRET) or photoinduced electron transfer reactions are still scarce. The replication of DNA by polymerase or telomerization of a nucleic acid by telomerase were monitored by the incorporation of a dye into the replica/ telomers associated with QDs and the use of FRET as readout signal.[7] The scission of duplex DNA linked to CdSe QDs by DNase [8] and the hydrolytic cleavage of peptides bound to CdSe QDs [9,10] were followed by FRET processes. Recently, the activities of tyrosinase and thrombin were analyzed by the tyrosinase-induced generation of quinone residues on amino acid or peptide capping layers associated with CdSe QDs. [11] This resulted in electron-transfer quenching of the QDs. The subsequent hydrolytic cleavage of the peptide by thrombin removed the quencher and recovered the fluorescence of the QDs.We describe the synthesis of Nile-blue-functionalized CdSe/ZnS quantum dots as a hybrid material that optically senses 1,4-dihydronicotinamide adenine dinucleotide (phosphate) cofactors, NAD(P)H. The modified quantum dots enable the fluorescence imaging of 1,4-nicotinamide adenine dinucleotide (phosphate) {NAD(P) + }-dependent biocatalytic transformations and allow the monitoring of the intracellular metabolism in HeLa cancer cells. This technique allows the application of the NAD(P)H-sensitive QDs to screen anticancer agents and to probe the effect of drugs on intracellular metabolism.Whereas previous applications of QDs to probe enzyme activities required the synthesis of specifically functionalized QDs, we sought generic functionalized QDs that could act as versatile probes to analyze different biocatalyzed transformations. Numerous redox enzymes use the common NAD(P) + cofactor, and hence the use of appropriately functionalized QDs to analyze NAD(P)H could provide a generic method to analyze NAD(P)+ -dependent enzymes, as well as to detect their substrates. Indeed, substantial efforts have been directed to the development of biosensors based on NAD(P)+ -dependent biocatalysts.[12]Different enzyme electrodes for the amperometric detection of the substrates of NAD(P) + -dependent enzymes were designed, and molecular electron relays [13] or redox polymers [14,15] were used to electrocatalyze the oxidation of NAD(P)H. Also, different integrated electrodes consisting of surface-confined relay-NAD(P) + -enzyme assemblies for the electrochemical analysis of different substrates were developed. [16,17] Recently, the NAD(P)H-stimulated growth of Au nanoparticles was used to develop optical sensors that probe NAD + -dependent enzymes and their substrates in solution or on surfaces.[18] Similarly, the NADH-mediated growth of ...