Detection of non-cross-linking interaction between DNA-modified gold nanoparticles and a DNA-modified flat gold surface using surface plasmon resonance imaging on a microchip

Gold nanoparticles (GNPs) with fully matched DNA duplexes on their surfaces aggregate together without molecular cross-linking at high salt concentrations. The mechanism of this non-cross-linking (NCL) interaction has been elusive. In this paper, NCL interaction between duplex-modified GNPs and a duplex-modified flat gold surface is presented for the first time. This new experimental platform has enabled us to study the NCL interaction between duplexes with different sequences. We immobilized 15-base single-stranded (ss) DNA onto the surfaces of GNPs with a diameter of 40nm and onto a flat gold substrate. The GNPs were hybridized with 15-base ssDNA at a low salt concentration. A microfluidic device was used for simultaneous delivery of the following three components onto the gold substrate: the duplex-modified GNPs, 15-base ssDNA to be hybridized onto the substrate, and NaCl at a high concentration. Adsorption of the GNPs onto the substrate was monitored using surface plasmon resonance imaging. When the GNPs and the substrate had an identical sequence, the adsorption behavior was analogous to the aggregation behavior of GNPs in test tubes. Furthermore, we investigated 12 cases in which the GNPs and the substrate had completely different sequences, and obtained results suggesting that the NCL attraction force primarily depends on the terminal base pairs of the duplexes. This means that the main mechanism of the NCL interaction is likely to be inter-duplex base stacking rather than formation of Holliday junctions.     

具有温度响应壳层的金纳米粒子的制备

利用可逆-加成断裂链转移聚合得到全亲水性的嵌段共聚物(PEO-b-PNIPAM), 通过"grafting to"使其接枝到金纳米粒子表面. 通过透射电子显微镜、 紫外-可见吸收光谱、 能谱分析及动态光散射研究了杂化的金纳米粒子的壳层结构及温度响应行为. 实验结果表明, 得到核壳结构的金纳米粒子, 同时其壳层具有温度响应行为. 随着温度的升高, 其流体力学半径略有减小. 在整个升温过程中, 由于外层PEO链段的抑制作用, 没有发生粒子间的聚集.     

G-rich oligonucleotide-functionalized gold nanoparticle aggregation

Guanine-rich DNA sequences commonly form helical quadruplex structures via Hoogsteen hydrogen bonds. The aggregation behavior of the nanoparticles, which are functionalized with four-guanine-terminated 27-base sequences at a nanoparticle-to-DNA ratio of 1:60, is investigated. To some extent, the guanine-quadruplex structures between the gold nanoparticles (GNPs) promote nanoparticle aggregation. However, the coordination site of the metal ion on the nanoparticle surface is partially passivated: the stability of guanine-rich DNA-GNPs is slightly lower than that of the usual DNA-GNPs, and the metal-ion specificity of nanoparticle assembly is substantially decreased. Thus, a mechanism for the aggregation of guanine-rich sequence-modified GNPs is proposed. It is possible to obtain a stable guanine-rich sequence-functionalized nanoparticle solution at high ionic strength by regulating guanine-rich DNA sequences. The controllability of guanine-rich sequence-modified nanoparticles makes the secondary structure of DNA a potentially useful candidate for DNA analysis and disease diagnostics. Figure Proposed mechanism for the aggregation of G-rich sequence-functionalized GNP Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Effect of bidispersity in grafted chain length on grafted chain conformations and potential of mean force between polymer grafted nanoparticles in a homopolymer matrix

In efforts to produce polymeric materials with tailored physical properties, significant interest has grown around the ability to control the spatial organization of nanoparticles in polymer nanocomposites. One way to achieve controlled particle arrangement is by grafting the nanoparticle surface with polymers that are compatible with the matrix, thus manipulating the interfacial interactions between the nanoparticles and the polymer matrix. Previous work has shown that the molecular weight of the grafted polymer, both at high grafting density and low grafting density, plays a key role in dictating the effective inter-particle interactions in a polymer matrix. At high grafting density nanoparticles disperse (aggregate) if the graft molecular weight is higher (lower) than the matrix molecular weight. At low grafting density the longer grafts can better shield the nanoparticle surface from direct particle-particle contacts than the shorter grafts and lead to the dispersion of the grafted particles in the matrix. Despite the importance of graft molecular weight, and evidence of non-trivial effects of polydispersity of chains grafted on flat surfaces, most theoretical work on polymer grafted nanoparticles has only focused on monodisperse grafted chains. In this paper, we focus on how bidispersity in grafted chain lengths affects the grafted chain conformations and inter-particle interactions in an implicit solvent and in a dense homopolymer polymer matrix. We first present the effects of bidispersity on grafted chain conformations in a single polymer grafted particle using purely Monte Carlo (MC) simulations. This is followed by calculations of the potential of mean force (PMF) between two grafted particles in a polymer matrix using a self-consistent Polymer Reference Interaction Site Model theory-Monte Carlo simulation approach. Monte Carlo simulations of a single polymer grafted particle in an implicit solvent show that in the bidisperse polymer grafted particles with an equal number of short and long grafts at low to medium grafting density, the short grafts are in a more coiled up conformation (lower radius of gyration) than their monodisperse counterparts to provide a larger free volume to the longer grafts so they can gain conformational entropy. The longer grafts do not show much difference in conformation from their monodisperse counterparts at low grafting density, but at medium grafting density the longer grafts exhibit less stretched conformations (lower radius of gyration) as compared to their monodisperse counterparts. In the presence of an explicit homopolymer matrix, the longer grafts are more compressed by the matrix homopolymer chains than the short grafts. We observe that the potential of mean force between bidisperse grafted particles has features of the PMF of monodisperse grafted particles with short grafts and monodisperse grafted particles with long grafts. The value of the PMF at contact is governed by the short grafts and values at large inter-particle distances are governed by the longer grafts. Further comparison of the PMF for bidisperse and monodisperse polymer grafted particles in a homopolymer matrix at varying parameters shows that the effects of matrix chain length, matrix packing fraction, grafting density, and particle curvature on the PMF between bidisperse polymer grafted particles are similar to those seen between monodisperse polymer grafted particles.     

Unmodified "GNP-oligonucleotide" nanobiohybrids: a simple route for emission enhancement of DNA intercalators

We present herein a simple method for enhancing the emission of DNA intercalators in homogeneous nanobiohybrids of unlabeled oligonucleotides and unmodified gold nanoparticles (GNPs). Pristine single‐stranded DNA (ss‐DNA) has been wrapped around unmodified GNPs to induce metal‐enhanced fluorescence (MEF) of DNA intercalators, such as ethidium bromide and propidium iodide. The thickness of the ss‐DNA layer on the gold nanosurface determines the extent of MEF, since this depends on the position of the intercalator in relation to the metal surface. Presumably, at a suitable thickness of this DNA layer, more of the intercalator is localized at the optimum distance from the nanoparticle to give rise to MEF. Importantly, no external spacer or coating agent was needed to induce the MEF effect of the GNPs. The concentration ratios of Au to DNA in the nanohybrids, as well as the capping agents applied to the GNPs, play key roles in enhancing the emission of the intercalators. The dimensions of both components of the nanobiohybrids, that is, the size of the GNPs and the length of the oligonucleotide, have considerable influences on the emission enhancement of the intercalators. Emission intensity increased with increasing size of the GNPs and length of the oligonucleotide only when the DNA efficiently wrapped the nanoparticles. An almost 100 % increment in the quantum yield of ethidium bromide was achieved with the GNP–DNA nanobiohybrid compared with that with DNA alone (in the absence of GNP), and the fluorescence emission was enhanced by 50 % even at an oligonucleotide concentration of 2 nM . The plasmonic effect of the GNPs in the emission enhancement was also established by the use of similar nanobioconjugates of ss‐DNA with nonmetallic carbon nanoparticles and TiO₂ nanoparticles, with which no increase in the fluorescence emission of ethidium bromide was observed.     

One-Step Synthesis of Stable Colloidal Gold Nanoparticles Through Bioconjugation with Bovine Serum Albumin in Harsh Environments

In saline solutions with NaCl concentrations less than that typical of blood plasma and bodily fluids, gold nanoparticles (GNPs) aggregate and precipitate because of GNP cation complexation with the Cl^? anions in the solution. It is difficult to retain stable colloidal GNPs within any saline solution for a relatively long time without aggregation and precipitation. In this study, we developed a method to synthesize stable GNPs in harsh anion-containing environments. GNPs were formed by laser ablation in a saline solution, and their stabilization was achieved by adding bovine serum albumin (BSA) to the NaCl solution; this has been shown to be a quick, efficient approach to producing stable colloidal GNPs. GNP nanoclusters in saline solutions with and without BSA were observed via high-resolution transmission electron microscopy and selected area electron diffraction. The results reveal that our methodology yields colloidal GNPs with long-term stability in a BSA-containing saline solution.     

Cellular Uptake of Gold Nanoparticles and Their Movement in 3D Multicellular Tumor Spheroids: Effect of Molecular Weight and Grafting Density of Poly(2‐hydroxyl ethyl acrylate)

It is known that the size of gold nanoparticles (GNPs) is not the only determining factor in the uptake by cells such as cancer cells. The surface functionalization plays a crucial role, in particular the nature of the ligand as well as the molecular weight and the grafting density. Here, poly(2‐hydroxy ethyl) acrylate (pHEA) with molecular weights ranging from 10, 20 to 39 g mol^?1 via reversible addition–fragmentation chain transfer polymerization is synthesized. These polymers are used directly to coat GNPs with sizes of 20, 40, and 70 nm as the trithiocarbonate functionality can strongly bind to the gold surface. The library of nine GNP is found to be nontoxic against lung carcinoma cells A549 and has negligible albumin protein absorption as determined by quartz crystal microbalance. Laser scanning confocal microscopy and flow cytometry reveal that GNP coated with medium length pHEA displays the highest cellular uptake while the effect of the size is not statistically significant. In contrast, multicellular tumor spheroids, which is a 3D model that simulates the tissue, enable the penetration of GNP coated with the longest pHEA chain while it also appears that smaller GNPs have now a clear advantage.     

Two-dimensional self-organization of polystyrene-capped gold nanoparticles

Thiol end-functionalized polystyrene chains have been introduced onto the surface of gold nanoparticles via a two-step grafting-to method. This simple grafting procedure is demonstrated to be efficient for gold nanoparticles of different sizes and for particles initially dispersed in either aqueous or organic media. The method has been applied successfully for a relatively large range of polystyrene chain lengths. Grafting densities, as determined by thermogravimetric analysis, are found to decrease with increasing chain length. In all cases, the grafting density indicates a dense brush conformation for the tethered chains. The resulting functionalized nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solution. Furthermore, the distance between the gold cores in the dried monolayer is controlled by the molecular weight of the grafted polystyrene. Optical absorption spectra recorded for the organized monolayers show the characteristic plasmon absorption of the gold particles. Importantly, the plasmon resonance frequency exhibits a distinct dependence on interparticle separation that can be attributed to plasmon coupling between neighboring gold cores.     

Enhancement of the colorimetric sensitivity of gold nanoparticles with triethanolamine to minimize interparticle repulsion

Cysteine and thioglycolic acid were immobilized on gold nanoparticles via established thiolgold surface chemistry. It is found that calcium ions rapidly induce the aggregation of the functional gold nanoparticles due to the complexation of Ca(II) by immobilized cysteine. It was also found that triethanolamine enhances the effect of calcium ions by decreasing the electrostatic repulsion between the gold nanoparticles. Transmission electron microscopy, electrophoresis, zeta potential measurements and absorptiometry were used to investigate the mechanism. Under the optimum experimental condition, the cysteine/thioglycolate/triethanolamine-modified nanoparticles were highly sensitive (the detection limit being 0.3 ??M) and selective towards calcium and magnesium ions, with a linear detection range between 1.0 ??M and 14 ??M. Based on these findings, a rapid and selective colorimetric method was developed for assaying Ca(II) ions in serum.

Ultrathin gold film deposited on human hair: Derivation from nanoparticles and applications as microsensors

An ultrathin film of gold was grafted on human hair by a chemical liquid deposition method under ambient conditions. The method consisted of the assembling of gold nanoparticles (GNPs) on the adsorptive sites of human hair as seeds and the growth of isolated GNP seeds into a continuous gold layer. The resulting gold film coated hair possessed good conductivity and flexibility, and can be used as a novel gold hair microelectrode (GHME). This electrode inherited some merits of both hair and gold nanoparticles, for instance, good mechanical property, excellent biocompatibility and high surface area. GHME was also proven to exhibit sensitive electrochemical responses toward dopamine and nitric oxide, foreseeing its promising applications in the fields of biomedical analysis.     

Novel Method to Graft Chitosan on the Surface of Hydroxyapatite Nanoparticles via ＂Click＂ Reaction

In order to tune the surface properties of hydroxyapatite（HA） nanoparticles and prevent them from ag- gregation, an efficient method was proposed to graft chitosan（CS） molecules on the surface of HA via ＂click＂ reac- tion. Thermal gravimetric analysis（TGA） shows that CS was successfully grafted on the surface of HA nanoparticles and the grafting amount was about 8.9 g of CS on per hundred grams of HA. The grafted chitosan chains can prevent HA nanoparticles from aggregation and greatly enhance the colloidal stability of HA in water. The 3-（4,5-dimethylthiazoyl-2-yl）-2,5-diphenyltetrazolium bromide（MTT） assay demonstrats that the cytotoxicity of CS modified HA（HA-CS） is negligible and thus HA-CS may find potential application in biomedical fields.     

SURFACE ACTIVITY AND ENERGETICS OF AGGREGATE FORMATION IN AQUEOUS SODIUM BENZOATE AND SALICYLATE SOLUTIONS

Aqueous solutions of the two hydrotropic agents sodium benzoate and sodium salicylate exhibit moderate surface activity as surface tension measurements indicated. The surface cross sectional area of the salicylate ion is reasonably higher than that of benzoate. The area occupied per ion increases with the rise in temperature for both hydrotropes. The critical aggregate concentration (CAC) obtained from viscosity and conductivity measurements substantiates those obtained from surface tension measurements. Analysis of the temperature dependence of CAC showed that aggregation of benzoate and salicylate ions in aqueous solution is driven by enthalpy and entropy factors. The entropy contribution is larger especially for salicylate ion. This indicates that the entropy gain associated with freeing water molecules structured around salicylate ions is the primary driving force for aggregation. The enthalpy contribution to aggregate formation of benzoate ion is relatively larger than that of salicylate, and this agrees well with more hydrophobic nature of the former species.     

Kinetics of receptor directed assembly of multisegment nanowires

We demonstrate the receptor directed end-to-end assembly of multisegment Au/Ni/Au nanowires under agitation in ethanol. The gold end-segments were functionalized with biotin-terminated thiol thereby restricting aggregation to end-to-end attachment via an avidin linkage. On mixing biotin-terminated nanowires with avidin-terminated nanowires, the average chain length is shown to increase linearly with time. The rate constant was independent of the nanowire concentration. Kinetic Monte Carlo simulations were used to model the self-assembly process, and we show that the directed end-to-end assembly of nanowires is similar to the polycondensation of linear polymers.     

Revisiting the dispersion mechanism of grafted nanoparticles in polymer matrix: a detailed molecular dynamics simulation

By focusing on the grafted nanoparticles (NPs) embedded in polymer melts, a detailed coarse-grained molecular dynamics simulation is adopted to investigate the effects of the grafting density, the length of the matrix and grafted chains on the dispersion of the NPs. We have employed visualization snapshots, radial distribution functions (RDFs), the interaction energy between NPs, the number of neighbor NPs, and the conformation of the brush chains to clearly analyze the dispersion state of the grafted NPs. Our simulated results generally indicate that the dispersion of the NPs is controlled by both the excluded volume of the grafted NPs and the interface between the brushes and the matrix. It is found that increasing grafting density or grafted chain length leads to better dispersion, owing to larger excluded volume; however, increasing the length of the matrix chains leads to aggregation of NPs, attributed to both a progressive loss of the interface between the brushes and the matrix and the overlap between brushes of different NPs, intrinsically driven by entropy. Meanwhile, it is found that there exists an optimum grafting density (σ(c)) for the dispersion of the NPs, which roughly obeys the following mathematical relation: σ(c) is proportional to N(m)(K)/N(g)(L), where K, L > 0 and N(m) and N(g) represent the length of the matrix and grafted chain length, respectively. Considering the practical situation that the grafted brushes and the matrix polymer are mostly not chemically identical, we also studied the effect of the compatibility between the brushes and the matrix polymer by taking into account the attraction between the grafted chains and the matrix chains. In general, our comprehensive simulation results are believed to guide the design and preparation of high-performance polymer nanocomposites with good or even tailored dispersion of NPs.     

Surface modification of magnetic metal nanoparticles and its influence on the performance of polymer composites

To control the interfacial interaction in magnetic metal nanoparticles‐filled polymer composites, surfaces of iron, cobalt, and nickel nanoparticles were grafted by irradiation‐induced polymerization. On the basis of the study of dynamical mechanical behavior, thermal stability, and magnetic performance of the composites prepared by either solution mixing or in situ polymerization, the structure–property relationships of the composites are a function of interfacial interaction and the dispersion state of the nanoparticles. In addition, grafting of polymers onto the surface of the metal nanoparticles changed the surface magnetic state, leading to the possibility of purposely tailoring the magnetic behavior of the composites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1070–1084, 2003     

Disclosure of the imidazolium cation coordination and stabilization mode in ionic liquid stabilized gold(0) nanoparticles

A surface-enhanced Raman spectroscopy (SERS) study of imidazolium ionic liquid stabilized gold(0) nanoparticles (GNPs) furnished previously unknown knowledge about the coordination and stabilization mode of the imidazolium cation. GNPs were prepared by hydrazine reduction of a chloroauric acid solution in 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate 2 as ether-functionalized room-temperature ionic liquid (RTIL). UV-vis spectroscopy showed the presence of GNP aggregates as absorptions extended to the NIR region. A parallel coordination mode for the imidazolium cation of RTIL 2 on the GNP surface was observed by SERS, which occurred without the simultaneous coordination of the 1-triethylene glycol monomethyl ether-functionality. Instead of this, the ether-functionality was directed away from the GNP surface and acted as steric barrier between the GNPs/GNP aggregates, thus preventing further aggregation. These new insights suggest that the imidazolium cation is responsible for electrosteric stabilization.     

Gold glyconanoparticles for mimics and measurement of metal ion-mediated carbohydrate-carbohydrate interactions

To mimic and measure calcium ion-mediated carbohydrate-carbohydrate interactions, four lactose derivatives have been synthesized for assembly on gold nanoparticles. The series of lactose derivatives varied by the length of the thiolated ethylene glycol anchor chain [O(CH2CH2O)(m)CH2CH2SH; where m = 0, 1, 2, and 3] used to self-assemble the carbohydrates to the preformed gold nanoparticles of ca. 16 nm diameter. Upon addition of calcium ions to the lactose-stabilized nanoparticles, rapid carbohydrate-carbohydrate interactions were visualized and subsequently measured using UV-visible spectrometry and transmission electron microscopy (TEM). The nanoparticle aggregates formed via metal-mediated carbohydrate-carbohydrate interactions could be readily redispersed through the addition of EDTA. Multiple reaggregation and redispersion cycles were achieved, confirming that the aggregation process was due to metal ion-mediated carbohydrate interactions rather than calcium chelation by residual citrate ions on the particle surface. The essential involvement of the lactose moiety in Ca2+ complexation was shown by control measurements on related D-glucose-derivatized nanoparticles, where a significantly reduced aggregation response was obtained only at high ion concentrations. Other group 2 metal ions with radii larger than that of calcium, viz., barium and strontium, were also shown to mediate the aggregation of the lactose-stabilized nanoparticles. The induced aggregation of the lactose nanoparticles was determined to be quantitatively dependent upon the calcium ion concentration. Furthermore, the analytical sensitivity of the calcium-induced aggregation and the linear dynamic range were dependent on the length of the ethylene glycol anchor chain. The shortest ethylene glycol chain (m = 0) gave the most sensitive response with the optimum limit of detection (0.8 mM Ca2+), whereas the longest ethylene glycol chain (m = 3) provides a measurement of calcium ion concentration over the largest linear dynamic range (10-35 mM Ca2+). This work has shown that the self-assembled deposition of lactose derivatives on gold nanoparticles provides multivalent carbohydrate surfaces that can be used as mimics for the measurement of biologically relevant carbohydrate-carbohydrate interactions. Additionally, this study has highlighted the importance of the structure and length of the ligand that anchors the carbohydrate sugar to the gold particle surface to facilitate such carbohydrate interactions and for "tuning" the analytical characteristics of bioassays developed using metal nanoparticle technology.     

Label-free detection of amino acids using gold nanoparticles in electrokinetic chromatography-thermal lens microscopy

To achieve label-free detection of amino acids in capillary-based electrokinetic chromatography-thermal lens microscopy (EKC-TLM), gold nanoparticles (GNPs), which possess the absorption around 500nm attributed to surface plasmon resonance (SPR), were added to the background solution (BGS). Since the SPR absorption of the GNPs exhibits a sensitive response toward environmental changes and the degree of aggregation, the sensitive detection of non-absorbing species is expected by using the GNPs in EKC-TLM (GNP-EKC-TLM). In the GNP-EKC-TLM analysis of glutamic acid (Glu), a sharp peak was observed when the GNPs were added to the BGS. The plot of the peak area of Glu against its concentration gave a good linear relationship and the limit of detection was estimated to be 25mug/mL. Furthermore, a baseline separation of lysine and Glu was successfully achieved. Thus, the EKC separation and label-free TLM detection of the amino acids can be realized only by adding the GNPs into the BGSs.     

An approach for the surface functionalized gold nanoparticles with pH-responsive polymer by combination of RAFT and click chemistry

In this report, we demonstrated a novel efficient post-modification route for preparation of smart hybrid gold nanoparticles with poly(4-vinylpyridine) (P4VP) based on RAFT and click chemistry. A new azide terminated ligand was first synthesized to modify gold nanoparticles by ligand exchange reaction, and then click reaction was used to graft alkyne terminated P4VP which was prepared by RAFT onto the surface of gold nanoparticles. The functionalized hybrid gold nanoparticles were characterized by TEM, FTIR, and XPS etc. The results indicated that the P4VP was successfully grafted onto the surface of gold nanoparticles by click reaction. The surface grafting density was calculated to be about 6 chains/nm²_. In addition, the hybrid gold nanoparticles showed a pH responsive phenomenon as the pH value changed around 5.     

Synthesis and characterization of polystyrene-grafted magnetite nanoparticles

Magnetite (Fe₃O₄) nanoparticles were synthesized by chemical precipitation. To reduce the aggregation of Fe₃O₄ nanoparticles, an effective surface modification method was proposed by grafting polystyrene onto the Fe₃O₄ particles. The results of Fourier transform infrared spectra and elemental analysis showed that the polymer chains have been successfully grafted from the surface of the Fe₃O₄ nanoparticles and that the percentage of grafting can reach 73%. Transmission electron microscope showed that grafted polymer chains on nanoparticles could prevent the aggregation of Fe₃O₄ nanoparticles markedly in toluene and improve their compatibility with organic phase. Another finding was the grafting reaction did not alter the crystalline structure of the Fe₃O₄ nanoparticles according to the X-ray diffraction patterns, and the saturation magnetization of PS-Fe₃O₄ nanoparticles was found to be lower than bulk magnetite.