Unique Size-dependent Optical Properties Research Articles

Performance comparison of surface plasmon resonance biosensors based on ultrasmall noble metal nanoparticles templated using bovine serum albumin

Due to their excellent chemical stability, biocompatibility, ease of modification and unique optical properties, ultrasmall noble metal nanoparticles (size 1–2 nm) are finding increasing application in the biomedical sector, in particular as cell markers and bioimaging agents. Herein, the unexplored potential of ultrasmall noble metal nanoparticles as sensitizers in surface plasmon resonance (SPR) biosensors is critically examined. Three types of bovine serum albumin (BSA) coated ultrasmall noble metal nanoparticles were synthesized (BSA-Au NPs, BSA-Au/Ag NPs and BSA-Ag NPs), with the effect of these different types of nanoparticles on the sensitivity of a SPR biosensor for the detection of the enzyme papain explored. Results show that the sensitivity of SPR biosensors containing BSA-Au NPs, BSA-Ag NPs or BSA-Au/Ag NPs BSA were 1.9, 2.6 and 3.6 times higher, respectively, than a control SPR biosensor (BSA only). The remarkable sensitizing properties of the ultrasmall noble metal nanoparticles, especially the BSA-Au/Ag NPs, related to their unique size dependent optical properties.

Suppression of Blinking and Enhancement of Optical Properties of Core-Shell Quantum Dots by Structural Formulation

Core-shell quantum dots (QDs) have gained immense significance in various technological applications due to their unique size-dependent optical properties. However, their efficiency in lightning and display applications can be limited by fluorescence intermittency or blinking which limits the viability of the QDs. In this work, a theoretical study on the causes and mitigation process of the blinking phenomenon in core-shell QDs has been carried out. An approach to eliminate blinking behavior is to modify the traditional core-shell structure where the interface between the core and shell materials is discontinuous and abrupt. A graded alloy core-shell QD where there is a gradual transition between pure core material and pure shell material shows suppressed blinking behavior. Another alternate strategy is the growth of core-shell QDs with giant shell structures and appropriate surrounding matrix considering the traps being few nanometers away from the core. The study of the effect of fluctuations in the local environment of the QDs helps in understanding the mechanisms of blinking and hence in curtailing its effect. These approaches have been applied and analyzed to suppress the blinking phenomenon and improve the optical properties of QDs.

Distribution of boron and phosphorus and roles of co-doping in colloidal silicon nanocrystals

Boron (B) and phosphorous (P) co-doped colloidal silicon nanocrystals (Si NCs) have unique size-dependent optical properties, which lead to potential applications in optoelectronic and biomedical applications. However, the microstructure of the B and P co-doped colloidal Si NCs – in particular, the exact location of the dopant atoms in real space, has not been studied. A lack of understanding of this underlying question limits our ability to better control sample fabrication, as well as our ability to further develop the optical properties. To study the microstructure, a process enabling atom probe tomography (APT) of colloidal Si NCs was developed. A dispersion of colloidal Si NCs in a SiO2 sol-gel solution and a low temperature curing are demonstrated as the key sample preparation steps. Our APT results demonstrate that a B-rich region exists at the surface of the Si NCs, while P atoms are distributed within the Si NCs. First principles density functional theory calculations of a Si NC embedded in SiO2 matrix reveal that P atoms, which always prefer to reside inside a Si NC, significantly influence the distribution of B atoms. Specifically, P atoms lower the B diffusion barrier at Si/SiO2 interface and stabilize B atoms to reside within individual Si NCs. We propose that the B-modified surface changes the chemical properties of the Si NCs by (i) offering chemical resistance to attack by HF and (ii) enabling dispersibility in solution without aggregation.

Size-Selective Optical Printing of Silicon Nanoparticles through Their Dipolar Magnetic Resonance

Silicon nanoparticles possess unique size-dependent optical properties due to their strong electric and magnetic resonances in the visible range. However, their widespread application has been limited, in comparison with other (e.g., metallic) nanoparticles, because their preparation on monodisperse colloids remains challenging. Exploiting the unique properties of Si nanoparticles in nano- and microdevices calls for methods able to sort and organize them from a colloidal suspension onto specific positions of solid substrates with nanometric precision. We demonstrate that surfactant-free silicon nanoparticles of a predefined and narrow (σ < 10 nm) size range can be selectively immobilized on a substrate by optical printing from a polydisperse colloidal suspension. The size selectivity is based on differential optical forces that can be applied on nanoparticles of different sizes by tuning the light wavelength to the size-dependent magnetic dipolar resonance of the nanoparticles.

Few-Nanometer-Sized α-CsPbI3 Quantum Dots Enabled by Strontium Substitution and Iodide Passivation for Efficient Red-Light Emitting Diodes.

Cubic phase CsPbI3 quantum dots (α-CsPbI3 QDs) as a newly emerging type of semiconducting QDs hold tremendous promise for fundamental research and optoelectronic device applications. However, stable and sub-5 nm-sized α-CsPbI3 QDs have rarely been demonstrated so far due to their highly labile ionic structure and low phase stability. Here, we report a novel strontium-substitution along with iodide passivation strategy to stabilize the cubic phase of CsPbI3, achieving the facile synthesis of α-CsPbI3 QDs with a series of controllable sizes down to sub-5 nm. We demonstrate that the incorporation of strontium ions can significantly increase the formation energies of α-CsPbI3 QDs and hence reduce the structure distortion to stabilize the cubic phase at the few-nanometer size. The size ranging from 15 down to sub-5 nm of as-prepared stable α-CsPbI3 QDs allowed us to investigate their unique size-dependent optical properties. Strikingly, the few-nanometer-sized α-CsPbI3 QDs turned out to retain high photoluminescence and highly close packing in solid state thin films, and the fabricated red light emitting diodes exhibited high brightness (1250 cd m-2 at 9.2 V) and good operational stability (L50 > 2 h driven by 6 V). The developed cation-substitution strategy will provide an alternative method to prepare uniform and finely size-controlled colloidal lead halide perovskite QDs for various optoelectronic applications.

One-pot ligand-free synthesis of gold nanoparticles: The role of glycerol as reducing-cum-stabilizing agent

The synthesis of gold nanoparticles (AuNPs), especially by green methods, has attracted great attention due to their unique size-dependent optical properties, biocompatibility and diverse applications in various fields. The formation of stable AuNPs with control of size and shape often requires the use of a surfactant or some surface stabilizing ligands. Unfortunately, these stabilizing agents may hinder the catalytic activity and adversely affect the biocompatibility of the surfactant/AuNPs assembly. In this article, we report a one-pot, green synthesis of ligand-free AuNPs employing the eco-friendly glycerol both as reducing agent and stabilizing agent, thereby, greatly simplifying the synthesis process. The effect of glycerol:water (G:W) ratio, pH, temperature, dilution, ionic strength and aging on the stability and particle size distribution was studied. Stable AuNPs with reasonably good size distribution (8 ± 2 nm) were obtained which, at room temperature, showed no significant agglomeration or size change up to 04 days and only slight changes were seen after 08 days. The stability and size distribution of the particles was found to be dependent on the G:W ratio, temperature and pH of the reaction media. Owing to their clean surface, the synthesized ligand-free AuNPs exhibit advantage of lower energy requirement for glycerol electro-oxidation when compared to bulk Au and AuNPs-PVP materials. Such surfactant-free biocompatible AuNPs prepared using the eco-friendly glycerol may find useful applications in catalysis and biomedical applications.

Wide range tuning of the size and emission color of CH3NH3PbBr3 quantum dots by surface ligands

Organic-inorganic halide perovskite CH3NH3PbX3 (X= I, Br, Cl) quantum dots (QDs) possess the characters of easy solution-process, high luminescence yield, and unique size-dependent optical properties. In this work, we have improved the nonaqueous emulsion method to synthesize halide perovskite CH3NH3PbBr3 QDs with tunable sizes. Their sizes have been tailored from 5.29 to 2.81 nm in diameter simply by varying the additive amount of surfactant, n-octylamine from 5 to 120 μL. Correspondingly, the photoluminescence (PL) peaks shift markedly from 520 nm to very deep blue, 436 nm due to quantum confinement effect. The PL quantum yields exceed 90% except for the smallest QDs. These high-quality QDs have potential to build high-performance optoelectronic devices.

Studies on Energy Transfer Between Gold Nanoparticles and Quantum Dots

Introducion Colloidal semiconductor nanoparticles, or quantum dots (QDs), have attracted much attention due to their unique size dependent optical properties. QDs have great potential for many kinds of applications, such as light emitting diodes, displays, and bioimaging etc. These applications all require the physicochemical properties of a single QDs even in highly concentrated conditions. However, the concentration quenching due to the fluorescence resonant energy transfer (FRET) between nanoparticles is likely to happen in such situations. We demonstrate the distance dependence of the FRET by using gold nanoparticles (AuNPs) having a high absorption coefficient as an energy acceptor. As a method to suppress the FRET from the QDs to the AuNPs, we focused on the capping ligands of nanoparticles. Polystyrene having a thiol terminal with well-controlled molecular weight was synthesized and employed as capping ligands of the AuNPs to change the interparticle distance between the QDs and AuNPs. Of course, when the the QDs and AuNPs are separated to the extent that the energy transfer between these species are improbable, FRET between the QDs should be naturally suppressed. Experimental 1-dodecanethiol(DDT)-capped CdSe/CdS core/shell nanoparticles were synthesized following the existing protocols1 and used as QDs. Briefly, CdSe core nanoparticles were synthesized by the hot injection method, and then a CdS shell was formed on the CdSe core using cadmium oleate and 1-octanethiol as precursors. The AuNPs were synthesized in a toluene/water two phase system by using DDT and tetraoctylammonium bromide as capping ligands and surfactant, respectively.2 An aqueous solution containing hydrogen tetrachloroaurate were in contact with the toluene solution, and another aqueous solution containing sodium tetrahydridborate as a reducing agent was added with vigorous stirring. Polystyrenes with two different weight-average molecular weight of 3937 (PS3937) and 5307 (PS5307) were synthesized by the reversible addition-fragmentation chain transfer polymerization,3 which were then used in the synthesis of AuNPs as capping ligands instead of DDT. To compare the energy transfer rate between the nanoparticles, photoluminescence lifetime of the mixture of AuNPs and QDs in chloroform solution was measured. Then, the nanoparticles were coated on a glass substrates to facilitate the interaction between the nanoparticles in a non-solvent, closely packed condition. Results and Discussion Figure 1 shows photoluminescence decay curves of the mixture of DDT-capped CdSe/CdS NPs and AuNPs in chloroform solution (a) and on the glass substrates (b). For comparison, results obtained without AuNPs were also shown. Photoluminescence lifetimes were calculated from the decay curves by using the two- or three-exponential equations. Decrease in the photoluminescence lifetime indicates the quenching by the FRET. In the solution state, although the intense photoluminescence quenching was expected, the average lifetimes was only reduced to 77% (29.9 ns) relative to the original QDs (38.8 ns) when the DDT-capped AuNPs were added. When polystyrene-capped AuNPs were added, photoluminescence lifetime was almost unchanged (Fig. 1a, 41.3ns). On the other hand, the addition of DDT-capped AuNPs decreased the lifetime down to 20% (7.6 ns) of the original QDs (37.1 ns) when the same combination was measured in the solid state (Figure 1b). The photoluminescence quenching by the energy transfer became efficient since the two types of nanoparticles locate in close proximity each other. On the other hand, the use of PS3937-capped AuNPs instead of DDT-capped ones maintained relatively long photoluminescence lifetime (28.4ns, 77% of the original value), and PS5307-capped AuNPs did not reduce the photoluminescence lifetime of DDT-capped CdSe/CdS NPs at all. These results indicated that bulky polystyrenes maintained interparticle distance even in the solid state to the extent that the energy transfer is improbable. Finally, we employed the terminally-thiolated polystyrene as capping ligands of the QDs. The photoluminescence intensity was almost maintained even in the solid state due to the suppression of the energy transfer (date not shown). These results corresponded with the theory that the energy transfer efficiency by the FRET decreases with increase in the distance between the fluorophore and dye molecules or particles. References (1) O. Chen, J. Zhao, V. P. Chauhan, J. Cui, C. Wong, D. K. Harris, H. Wei, H. S. Han, D. Fukumura, R. K. Jain, and M. G. Bawendi, Nat Mater, 12, 445 (2013). (2) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, and R. Whyman, J. Chem. Soc., Chem. Commun., 801 (1994) (3) S.-K. Bae, S.-Y. Lee, and S. C. Hong, React. Funct. Polym., 71, 187 (2011). . Figure 1

One-step Synthesis of Water-dispersible ZnSe(S)-alloy Quantum Dots in the Presence of Thiol Species

Semiconductors quantum dots (QD) exhibit unique size-dependent optical properties which enable them to be considered for biomedical applications, including diagnosis and cancer treatment. Furthermore, the synthesis of less toxic and Cd-free QDs directly in aqueous phase assures their biocompatibility as required for nanomedicine applications. Accordingly, the present work was focused on the direct synthesis of alloyed ZnSe(S) QDs capped with thioglycolic acid (TGA) or 3-mercaptopropionic acid (MPA) in aqueous medium under microwave irradiation conditions. X-ray diffraction measurements suggested that as-synthesized QDs exhibited a solidsolution like structure with an average crystallite size of 3.8 ± 0.1 nm and 2.1 ± 0.2 nm for TGA- and MPA-capped Zn-based QDs, respectively. HRTEM measurements confirmed the small size and monodispersity of the synthesized QDs. The chemisorption of carboxylate groups in TGA (vas COO at 1577 cm-1 and vs COO at 1388 cm-1) and MPA (vas COO at 1560 cm-1 and vs COO at 1401 cm-1) onto the QDs was confirmed by FT-IR spectroscopy analyses. The band gap energy values were estimated at 3.1eV or 3.3eV, respectively, when the QDs were synthesized in the presence of TGA or MPA species. QDs suspended in biocompatible Phosphate Buffer Saline (PBS) exhibited a remarkable photo-stability; the corresponding photo-luminescence spectra recorded under 302nm excitation evidenced strong emission peaks centered on 390nm and 395nm for TGA- and MPA-capped QDs, respectively. As-synthesized thiol-capped alloyed ZnSe(S) QDs can be considered very promising biocompatible candidates for nanomedicine applications.

Improving Colorimetric Assays through Protein Enzyme-Assisted Gold Nanoparticle Amplification

The discovery of the DNA-mediated assembly of gold nanoparticles was a great moment in the history of science; this understanding and chemical control enabled the rational design of functional nanomaterials as novel probes in biodetection. In contrast with conventional probes such as organic dyes, gold nanoparticles exhibit high photostability and unique size-dependent optical properties. Because of their high extinction coefficients and strong distance dependent optical properties, these nanoparticles have emerged over the past decade as a promising platform for rapid, highly sensitive colorimetric assays that allow for the visual detection of low concentrations of metal ions, small molecules, and biomacromolecules. These discoveries have deepened our knowledge of biological phenomena and facilitated the development of many new diagnostic and therapeutic tools. Despite these many advances and continued research efforts, current nanoparticle-based colorimetric detection systems still suffer from several drawbacks, such as limited sensitivity and selectivity. This Account describes the recent development of colorimetric assays based on protein enzyme-assisted gold nanoparticle amplification. The benefits of such detection systems include significantly improved detection sensitivity and selectivity. First, we discuss the general design of enzyme-modified nanoparticle systems in colorimetric assays. We show that a quantitative understanding of the unique properties of different enzymes is paramount for effective biological assays. We then examine the assays for nucleic acid detection based on different types of enzymes, including endonucleases, ligases, and polymerases. For each of these assays, we identify the underlying principles that contribute to the enhanced detection capability of nanoparticle systems and illustrate them with selected examples. Furthermore, we demonstrate that the combination of gold nanoparticles and specific enzymes can probe enzyme dynamics and function with high specificity, offering substantial advantages in both sensitivity and specificity over conventional detection methods. The screening of nuclease, methyltransferase, protease, and kinase activities can be colorimetrically performed in a straightforward manner. Finally, we discuss examples of colorimetric assays for metal ions and small molecules that constitute important advances toward visual monitoring of enzyme catalytic functions and gene expression. Although these enzyme-assisted assay methods hold great promise for myriad applications in biomedicine and bioimaging, the application of the described techniques in vivo faces formidable challenges. In addition, researchers do not fully understand the interactions of gold nanoparticles with enzyme molecules. This understanding will require the development of new techniques to probe enzyme substrate dynamics at the particle interface with higher spatial resolution and chemical specificity.

Selective Synthesis of Strained [7]Cycloparaphenylene: An Orange-Emitting Fluorophore

[n]Cycloparaphenylenes, which are short fragments of carbon nanotubes, have unique size-dependent optical properties. In this communication, we describe the first synthesis of [7]cycloparaphenylene ([7]CPP), the smallest cycloparaphenylene prepared to date. In order to access this structure, we have developed a synthetic route that capitalizes on successive orthogonal Suzuki-Miyaura coupling reactions. [7]CPP has 83 kcal/mol of strain energy and an orange emission at 592 nm.

Potential clinical applications of quantum dots

The use of luminescent colloidal quantum dots in biological investigations has increased dramatically over the past several years due to their unique size-dependent optical properties and recent advances in biofunctionalization. In this review, we describe the methods for generating high-quality nanocrystals and report on current and potential uses of these versatile materials. Numerous examples are provided in several key areas including cell labeling, biosensing, in vivo imaging, bimodal magnetic-luminescent imaging, and diagnostics. We also explore toxicity issues surrounding these materials and speculate about the future uses of quantum dots in a clinical setting.