Interface effects and relaxation processes in nanocomposites based on CdSe/ZnS semiconductor quantum dots and porphyrin molecules

Controllable self-assembly and properties of nanocomposites based on CdSe/ZnS semiconductor quantum dots (QDs) and tetrapyridylporphyrin molecules (H₂P) as well as the dynamics of relaxation processes in these systems were studied for solutions and single nanoobjects in the temperature range of 77–295 K. It was proved that the formation of surface states of different nature is crucial to nonradiative relaxation of exciton excitation in QDs. The efficiency of QD→Н₂Р energy transfer was shown to be at most 10–15%. Regularities of photoluminescence (PL) quenching for QDs in nanocomposites in solutions of different polarity correlate with the dependences of PL blinking for single QDs. A scheme was proposed of excited states and main relaxation channels of exciton excitation energy in semiconductor QDs and QD–Н₂Р nanocomposites.     

Synthesis of amphiphilic triblock copolymers as multidentate ligands for biocompatible coating of quantum dots

One barrier to apply current tri-octylphosphine oxide (TOPO) based quantum dots (QDs) to biomedical imaging is that the TOPO on TOPO-QDs can be replaced by the proteins in living system, which may cause the degradation of QDs and/or deactivation of protein. In order to develop biocompatible optical imaging agents, a novel triblock copolymer, designed as a multidentate ligand, was synthesized to coat quantum dot nanocrystals (QDs). The copolymer consists of a polycarboxylic acid block at one end and a polythiol block at the other end with an intervening cross-linked poly(styrene-co-divinylbenzene) block bridging the ends. The multiple mercapto groups from the polythiol block act as multidentate ligands to stabilize QDs, while the polycarboxylic acid block improves the water solubility of QDs and offers reaction sites for surface modification or conjugation with bimolecules. The cross-linked poly(styrene-co-divinylbenzene) block provides a densely compacted hydrophobic shell. This shell will act as a barrier to inhibit the degradation of QDs by preventing the diffusion of ions and small molecules into the core of QDs. This new multidentate polymer coating facilitates the transfer of QDs from organic solvent into aqueous phase. The QDs directly bound to multidentate mercapto groups instead of TOPO are less likely to be affected by the mercapto or disulfide groups within proteins or other biomolecules. Therefore, this research will provide an alternative coating material instead of TOPO to produce QDs which could be more suitable for in vivo use under complex physiological conditions.     

Quantum Dot Photoluminescence Quenching by Cr(III) Complexes. Photosensitized Reactions and Evidence for a FRET Mechanism

Reported are quantitative studies of the energy transfer from water-soluble CdSe/ZnS and CdSeS/ZnS core/shell quantum dots (QDs) to the Cr(III) complexes trans-Cr(N(4))(X)(2)(+) (N(4) is a tetraazamacrocycle ligand, X(-) is CN(-), Cl(-), or ONO(-)) in aqueous solution. Variation of N(4), of X(-), and of the QD size and composition allows one to probe the relationship between the emission/absorption overlap integral parameter and the efficiency of the quenching of the QD photoluminescence (PL) by the chromium(III) complexes. Steady-state studies of the QD PL in the presence of different concentrations of trans-Cr(N(4))(X)(2)(+) indicate a clear correlation between quenching efficiency and the overlap integral largely consistent with the predicted behavior of a F?rster resonance energy transfer (FRET)-type mechanism. PL lifetimes show analogous correlations, and these results demonstrate that spectral overlap is an important consideration when designing supramolecular systems that incorporate QDs as photosensitizers. In the latter context, we extend earlier studies demonstrating that the water-soluble CdSe/ZnS and CdSeS/ZnS QDs photosensitize nitric oxide release from the trans-Cr(cyclam)(ONO)(2)(+) cation (cyclam = 1,4,8,11-tetraazacyclotetradecane) and report the efficiency (quantum yield) for this process. An improved synthesis of ternary CdSeS core/shell QDs is also described.     

金属离子对CdS量子点/聚酰胺-胺树形分子纳米复合材料光致发光性能的影响

以聚酰胺-胺树形分子为模板制备了分散好、尺寸均匀的CdS量子点，并用分光光度滴定法研究了Cd²⁺、Zn²⁺、Pb²⁺、Cu²⁺、Mn²⁺几种金属离子对其光致发光性能的影响。发现不同离子对CdS量子点的发光性能影响不同:Cd²⁺和Zn²⁺使量子点荧光增强，Pb²⁺、Cu²⁺和Mn²⁺使其荧光有不同程度淬灭。这归因于金属离子对CdS量子点表面的修饰作用。Cd²⁺能减少由S^2-悬键构成的非辐射复合中心，增强树形分子对量子点表面缺陷的钝化作用，并能在量子点周围形成类肖特基能垒，从而显著增大CdS量子点的光致发光效率。由于ZnS与CdS的晶格参数非常接近，Zn²⁺能起到与Cd²⁺类似的作用，使CdS量子点的发光效率大大增强。Pb²⁺和Cu²⁺能取代Cd²⁺在CdS量子点表面生成窄带隙的壳层，对其发光有很强的淬灭作用。由于块体PbS的带隙比块体CuS窄，故Pb²⁺的淬灭能力强于Cu²⁺。Mn²⁺能破坏Cd²⁺与PAMAM树形分子的配位键，降低树形分子对CdS量子点表面缺陷的钝化作用，且其本身在量子点表面构成了新的荧光淬灭中心，但Mn²⁺也能形成较弱的类肖特基能垒，故对量子点的发光淬灭作用较弱。     

Preparation and characterization of thiacalix[4]arene coated water-soluble CdSe/ZnS quantum dots as a fluorescent probe for Cu2+ ions

Highly fluorescent water-soluble CdSe/ZnS (core/shell) quantum dots (QDs) as a fluorescent Cu2+ ion probe were synthesized using thiacalix[4]arene carboxylic acid (TCC) as a surface coating agent. Hydrophobic trioctylphosphine oxide (TOPO) capped CdSe/ZnS QDs were overcoated with TCC in tetrahydrofuran at room temperature, and deprotonation of the carboxyl groups of TCC resulted in the formation of water-soluble QDs. The surface structure of the QDs was characterized by using transmission electron microscopy (TEM) and fluorescence correlation spectroscopy (FCS). TEM images showed that TCC-coated QDs were monodispersed with the particle size (core-shell moiety) of approximately 5 nm. Hydrodynamic diameter of the TCC-coated QDs was determined to be 8.9 nm by FCS, showing that the thickness of the surface organic layer of the QDs was approximately 2 nm. These results indicate that the surface layer of TCC-coated QDs forms a bilayer structure consisting of TOPO and TCC molecules. TCC-coated CdSe/ZnS QDs were highly fluorescent (quantum yield, 0.21) compared to the QDs surface-modified with mercaptoacetic acid and mercaptoundecanoic acid. Fluorescence of the TCC-coated QDs was effectively quenched by Cu2+ ions even in the presence of other transition metal ions such as Cd2+, Zn2+, Co2+, Fe2+, and Fe3+ ions in the same solution. The Stern-Volmer plot for the fluorescence quenching by Cu2+ ions showed a linear relationship up to 30 microM of Cu2+ ions. The ion selectivity of TCC-coated QDs was determined by measurements of fluorescence responses towards biologically important transition metal ions (50 microM) including Fe2+, Fe3+, Co2+>Zn2+, Cd2+. The fluorescence of TCC-coated QDs was almost insensitive to other biologically important ions such as Na+, K+, Mg2+, and Ca2+, suggesting that TCC-coated QDs can be used as a fluorescent Cu2+ ion probe for biological samples. A possible quenching mechanism by Cu2+ ions was also discussed on the basis of a Langmuir-type adsorption isotherm.     

溶液环境中CdSeS量子点与金纳米颗粒相互作用

选用不同发射波长的合金型CdSeS量子点(QDs), 研究溶液状态下量子点和金纳米颗粒(AuNPs)相互作用及离子强度、pH值、距离等诸多因素对相互作用的影响, 在此基础上对相互作用的机理进行了分析. 在溶液状态下, 金纳米颗粒可以高效地淬灭量子点, Stern-Volmer 淬灭常数K_sv值在108 L·mol^-1数量级. 这种淬灭效应与距离、光谱之间叠合程度等密切相关, 受溶液极性、离子强度、pH值的影响较小. 金纳米颗粒与量子点相互作用的机理较为复杂, 以能量转移为主. 研究结果对设计更高效的生物传感器及更全面认识金纳米颗粒与量子点相互作用的机理具有重要意义.     

Effect of CdS Interlayer on Properties of CdTe Based Quantum Dots

Highly luminescent thioglycolic acid-capped CdTe-based core/shell quantum dots (QDs) were synthesized through encapsulating CdTe QDs in various inorganic shells including CdS, ZnS and CdZnS. CdTe/CdS core/shell QDs exhibited a significant redshift of emission peaks (a maximum emission peak of 652 nm for the core/shell QDs and 575 nm for CdTe cores) with increasing shell thickness. In contrast, the redshift of photoluminescence (PL) peak wavelength of CdTe/ZnS QDs was less than 15 nm. The PL peak wavelengths of the core/shell QDs depended strongly on core size and shell thickness. The PL quantum yields (QYs) of the CdTe/CdS core/shell QDs are up to 67 % while that of CdTe/ZnS core/shell QDs is 45 %. A composite CdZnS shell made CdTe cores a high PL QY up to 51 % and broadly adjusted PL spectra (a maximum PL peak wavelength of 664 nm). The epitaxial growth of the shell was confirmed by X-ray powder diffraction analysis and luminescence decay experiments. Because of high PL QYs, tunable PL spectra, and low toxicity from a ZnS surface layer, CdTe/CdZnS core/shell QDs will be great potential for bioapplications.     

Quantum dot fluorescence quenching pathways with Cr(III) complexes. photosensitized NO production from trans-Cr(cyclam)(ONO)2+

Described is the photoluminescence (PL) of water-soluble CdSe/ZnS core/shell quantum dots (QDs) as perturbed by salts of the chromium(III) complexes trans-Cr(cyclam)Cl2+ (1), trans-Cr(cyclam)(ONO)2+ (2), and trans-Cr(cyclam)(CN)2+ (3) (cyclam = 1,4,8,11-tetraazacyclo-tetradecane). The purpose is to probe the characteristics of such QDs as antennae for photosensitized release of bioactive agents (in the present case, the bioregulatory molecule NO) from transition metal centers. Addition of 1 or 2 to a QD solution results in concentration-dependent quenching of the band edge emission, but 3 has a minimal effect. Added KCl strongly attenuates the quenching by 1, and this suggests that the Cr(III) cations and the QDs form electrostatic assemblies via ion pairing on the negatively charged QD surfaces. Quenching by 2, a known photochemical NO precursor, was accompanied by photosensitized NO release. All three, however, do quench the broad red emission ( approximately 650-850 nm) attributed to radiative decay of surface trapped carriers. The effect of various concentrations of 1 on time-resolved PL and absorbance were explored using ultrafast spectroscopic methods. These observations are interpreted in terms of the F?rster resonance energy-transfer mechanism for quenching of the band edge PL by multiple units of 1 or 2 at the QD surface, whereas quenching of the low-energy trap emission occurs via a charge-transfer pathway.     

Photoinduced fluorescence intensity oscillation in a reaction-diffusion cell containing a colloidal quantum dot dispersion

The nonlinear spontaneous oscillation of photoluminescence (PL) intensity in an ensemble of semiconductor quantum dots (QDs), which differs from the fluorescence intermittency of a single QD, is investigated. The PL intensity in a QD dispersion slowly oscillates with time under continuous illumination. The oscillatory behavior is found to vary with changing QD concentration, solvent viscosity, volume fraction of irradiated region, and irradiation intensity. On the basis of the Gray-Scott model [Chemical Oscillation and Instabilities: Non-linear Chemical Kinetics (Clarendon, Oxford, 1994); J. Phys. Chem. 89, 22 (1985); Chem. Eng. Sci. 42, 307 (1987)], and its comparison with the experimental results, it is revealed that the following processes are important for PL oscillation: (1) mass transfer of QDs between the illuminated and dark regions, (2) autocatalytic formation of vacant sites on QD surfaces via photodesorption of ligand molecules, and (3) passivation of vacant sites via photoadsorption of water molecules.     

Designer multi-functional comb-polymers for surface engineering of quantum dots on the nanoscale

Synthesis, characterization and cell-biomarker applications for a novel series of comb-copolymers featuring hydrophobic amide side groups, carboxylates and other functionalities such as polymerizable side chains and PEG oligomers are reported. When used as polymer ligand shell for trioctylphosphine oxide (TOPO) coated quantum dots (QDs) these copolymers effectively solubilise CdSe/ZnS QDs in water. A systematic study was carried out to find the relation between the molecular structure of the copolymers, their ability to coat the QDs, and to suspend the nanocrystals in water. To demonstrate potential applications, highly luminescent QD/polymer assemblies were internalized into living and fixed cells.     

New Preparation of Structurally Symmetric,Biodegradable Poly(L‐lactide) Disulfides and PLLA–Stabilized,Photoluminescent CdSe Quantum Dots

New, biodegradable poly(L ‐lactide) disulfides, PLLA‐SS‐PLLA, were first prepared through the DMAP‐catalyzed ring‐opening polymerization of L ‐lactide with a dihydroxyethyl disulfide initiator, and were further catalytically reduced into thiol‐end‐functionalized poly(L ‐lactide)s, HO‐PLLA‐SH, with a tributyl phosphine catalyst (PBu₃). Employing the HO‐PLLA‐SH as the ligand, new core‐shell CdSe/PLLA quantum dots (QDs) were continuously prepared via a facile ligand‐exchanging process with the CdSe/TOPO QD precursor. The chemical structures, morphologies and solvent solubility of these prepared CdSe/PLLA QDs were investigated by NMR spectroscopy, FTIR spectroscopy, XRD, TEM and excitation under either room light or UV radiation at 365 nm, demonstrating the successful ligand replacement and the new formation of core‐shell CdSe/PLLA QDs (diameter:4.0 ± 0.3 nm). Finally, UV and FL results indicate the two factors of the HO‐PLLA‐SH ligand molecular weight and the ligand/QD precursor feeding weight ratio were important for preparing stable and highly photoluminescent CdSe/PLLA QDs.

     

Pulsed field gradient NMR studies of polymer adsorption on colloidal CdSe quantum dots

Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments have been used to examine ligand exchange between poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMA) (Mn = 12,000, Mw/Mn = 1.20, Nn = 78) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). We show that PFG 1H NMR can quantify the displacement of TOPO by PDMA through its ability to differentiate signals due to TOPO bound to the QDs versus those from TOPO molecules free in solution. For CdSe QDs with a band edge absorption maximum at 558 nm (diameter 2.7 nm by transmission electron microscopy), we determined that, at saturation, 8 polymer chains on average displace greater than 90% of the surface TOPO groups. At partial saturation, with an average of 6 polymer chains/QD, each TOPO displaced requires 28 DMA repeat units. Assuming that one Me2N- group binds to a surface Cd2+ for each TOPO displaced, we infer that only about 3% of the DMA units are directly bound to the surface. The remaining groups are present as loops or tails that protrude into the solvent and increase the hydrodynamic diameter of the particles.     

Excitonic Chemiluminescence in Si and CdSe Nanocrystals Induced by their Interaction with Ozone

The results of a systematic study of spectral and kinetic patterns of ozone‐adsorption‐induced luminescence (AL) in nanostructured Si and, for the first time, in colloidal CdSe/ZnS quantum dots (QDs) are reported and compared with photoluminescence (PL) of the same structures. The common excitonic nature of light emission under ozone chemisorption and photoexcitation is confirmed by excellent coincidence of AL and PL emission bands. This coincidence is maintained during correlated quenching of AL and PL emission caused by nonradiative defects generated under ozone adsorption. A possible mechanism for energy conversion is proposed in the framework of an exothermic oxidation reaction of core materials caused by ozone. The significant role of the quantum confinement effect differentiates the observed phenomenon from well‐known chemiluminescence in molecular systems. This research establishes a physicochemical basis for the development of a gas sensor with high selectivity to ozone. Also, our findings may be useful for testing core–shell colloidal QDs with ozone.     

Nanoassemblies designed from semiconductor quantum dots and molecular arrays

The formation of nanoassemblies of CdSe/ZnS quantum dots (QD) and pyridyl-substituted free-base porphyrin (H(2)P) molecules has been spectroscopically identified by static and time-resolved techniques. The formation of nanoassemblies has been engineered by controlling the type and geometry of the H(2)P molecules. Pyridyl functionalization gives rise to a strong complex formation accompanied by QD photoluminescence (PL) quenching. For some of the systems, this quenching is partly related to fluorescence resonance energy transfer (FRET) from the QD to H(2)P and can be explained according to the F?rster model. The quantitative interpretation of PL quenching due to complexation reveals that (i) on average only about (1)/(5) of the H(2)P molecules at a given H(2)P/QD molar ratio are assembled on the QD and (ii) only a limited number of "vacancies" accessible for H(2)P attachment exist on the QD surface.     

Multidentate surface ligand exchange for the immobilization of CdSe/ZnS quantum dots and surface quantum dot-oligonucleotide conjugates

A method for synthesizing multidentate thiol ligands on fused silica surfaces (e.g., optical fibers) was developed for the immobilization of CdSe/ZnS quantum dots (QDs) capped with hydrophilic or hydrophobic ligands. This work was motivated by the poor stability of QDs immobilized via monodentate thiol ligands and the need for stable immobilization strategies in the development of sensor technologies based on QDs. Multi-dentate immobilization was able to withstand washing protocols, and surface ligand exchange occurred via self-assembly through the zinc-metal affinity interaction. Atomic force and scanning electron microscopy images suggested that the QDs were immobilized at high density, approximately 2-4 x 10 (13) cm (-2). It was possible to immobilize one, two, or three colors of QD. Upon immobilization, 1-2 nm bathochromic shifts in the PL spectra were observed. This was attributed to both ligand exchange and the change in local environment. The change in environment was accompanied by a decrease in PL lifetime. Self-assembly of immobilized QD-oligonucleotide and QD-avidin conjugates was also demonstrated. These conjugates were able to hybridize with complementary oligonucleotide and bind biotin, respectively. This versatile immobilization chemistry is an important step in the development of surface-based QD nanosensors. Such technology requires QDs to be immobilized such that they remain accessible to target molecules in solution.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

A Brief Overview of Some Physical Studies on the Relaxation Dynamics and Förster Resonance Energy Transfer of Semiconductor Quantum Dots

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) and the way these phenomena change with size, shape, and composition of the QDs. The understanding of the excited‐state dynamics of photoexcited QDs is essential for technological applications such as efficient solar energy conversion, light‐emitting diodes, and photovoltaic cells. Here, our emphasis is directed at describing the influence of size, shape, and composition of the QDs on their different relaxation processes, that is, radiative relaxation rate, nonradiative relaxation rate, and number of trap states. A stochastic model of carrier relaxation dynamics in semiconductor QDs was proposed to correlate with the experimental results. Many recent studies reveal that the energy transfer between the QDs and a dye is a FRET process, as established from 1/d⁶ distance dependence. QD‐based energy‐transfer processes have been used in applications such as luminescence tagging, imaging, sensors, and light harvesting. Thus, the understanding of the interaction between the excited state of the QD and the dye molecule and quantitative estimation of the number of dye molecules attached to the surface of the QD by using a kinetic model is important. Here, we highlight the influence of size, shape, and composition of QDs on the kinetics of energy transfer. Interesting findings reveal that QD‐based energy‐transfer processes offer exciting opportunities for future applications. Finally, a tentative outlook on future developments in this research field is given.     

Spectral and photochemical properties of hybrid organic—inorganic nanosystems based on CdS quantum dots and a styrylquinoline ligand

The kinetics of photolysis of a styrylquinoline (SQ) derivative as the photochromic ligand in organic—inorganic hybrid nanosystems (HNSs) with the core composed of CdS quantum dots (QDs) has been studied for the first time as a function of the number of ligand molecules in the HNS shell, which varied from 1 to 10. The hybrid nanosystems have been synthesized in the microwave-assisted mode according to the single-step injection-free procedure. It has been shown that high quantum yields of photoisomerization of the SQ ligand are conserved in the HNS. In the early stages of the photolysis, regardless of the number of SQ ligand molecules in the HNS shell, the kinetics obeys the equation for the photolysis of the monomolecular system (model SQ photochrome) with allowance for the absorption due to QDs as an inert shutter. During the course of long-term photolysis, the quantum dots undergo photodegradation to be completely decomposed. According to the principal component analysis data, several photoproducts with different absorption spectra are formed at the intermediate times of the HNS photolysis.     

Luminescence of Polyethylene Glycol Coated CdSeTe/ZnS and InP/ZnS Nanoparticles in the Presence of Copper Cations

The use of click chemistry for quantum dot (QD) functionalization could be very promising for the development of bioconjugates dedicated to in vivo applications. Alkyne–azide ligation usually requires copper(I) catalysis. The luminescence response of CdSeTe/ZnS nanoparticles coated with polyethylene glycol (PEG) is studied in the presence of copper cations, and compared to that of InP/ZnS QDs coated with mercaptoundecanoic acid (MUA). The quenching mechanisms appear different. Luminescence quenching occurs without any wavelength shift in the absorption and emission spectra for the CdSeTe/ZnS/PEG nanocrystals. In this case, the presence of copper in the ZnS shell is evidenced by energy‐filtered transmission electron microscopy (EF‐TEM). By contrast, in the case of InP/ZnS/MUA nanocrystals, a redshift of the excitation and emission spectra, accompanied by an increase in absorbance and a decrease in photoluminescence, is observed. For CdSeTe/ZnS/PEG nanocrystals, PL quenching is enhanced for QDs with 1) smaller inorganic‐core diameter, 2) thinner PEG shell, and 3) hydroxyl terminal groups. Whereas copper‐induced PL quenching can be interesting for the design of sensitive cation sensors, copper‐free click reactions should be used for the efficient functionalization of nanocrystals dedicated to bioapplications, in order to achieve highly luminescent QD bioconjugates.     

Fluorescence resonance energy transfer in CdSe/ZnS-DNA conjugates: probing hybridization and DNA cleavage

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.