Gold nanoparticles have been widely utilized to achieve colorimetric detection for various diagnostic applications. One of the most frequently used methods for DNA detection involves the aggregation of DNA-modified gold nanoparticles driven by target DNA hybridization. This process, however, is intrinsically slow, limiting its use in rapid diagnostics. Here we take advantage of the reverse process: the disassembly of preformed aggregates triggered by the addition of target DNA via a strand displacement mechanism. A systematic study of the dependence of the disassembly rate on temperature, with and without toeholds, has delivered a system that produces an extremely rapid colorimetric response. Furthermore, using an optimal toehold length of 5 nucleotides, target triggered disassembly is rapid over a wide range of ambient temperatures. Using this overhang system, simple visualization of low picomole amounts of target DNA is possible within 10 min at room temperature.

Polyvalent gold nanoparticle−DNA conjugates hybridize with complementary linker DNA strands to form aggregates that exhibit sharp dissociation curves indicative of cooperative behavior. Introducing single-stranded gaps consisting of thymidines (T 1 −T 20 ) into the linker strand resulted in a decrease in the number of duplexes that dissociate cooperatively. Upon adding one base insertion (T 1 ) the cooperative number drops from 6.3(2) to 2.8(2) duplexes. The cooperative number then increases slightly for the T 3 gap and thereafter decreases for T 8 and T 10 , with a slight increase again for the T 20 gap. As the presence of a shared condensed cation cloud has been implicated in neighboring duplex cooperativity, we measured the saltdependent behavior of T n gap-linked unmodified duplexes and the number of ions released per duplex dissociation. Interestingly, the number of cations released for the duplexes with a longer gap sequence is significantly larger than the number released for a T 1 gap-linked duplex or a nicked duplex (T 0 ). Overall there is a correlation between the change in condensed cation density and the dissociation entropy for the unmodified T n gap-linked duplexes, and the cooperative unit for the T n gap-linked GNP−DNA aggregates. Using dynamic light scattering and changes in optical absorbance, we also found that aggregation of GNP−DNA is more rapid when hybridization occurs at a nicked versus gap site, which was previously observed but attributed to slower hybridization as a result of the longer linker strand. By comparing the aggregation rate of a prehybridized GNP−DNA:T 10 -linker complex with a completely complementary GNP−DNA and a GNP−DNA that led to a T 10 gap, we were able to establish that the presence of the gap, not DNA length or accessibility, caused the decrease in aggregation rate. Our results support that flexibility in aggregates decreases the rate of aggregation as well as the extent of cooperativity, which has important implications in genomic DNA detection.

Isolating the contribution of silica in second harmonic generation (SHG) studies at the silica/water interface remains a challenge. Herein, we compare SHG intensities with previously measured zeta potentials and vibrational sum frequency generation (SFG) intensities to deconvolute the silica contribution in the SHG measurements. Under conditions that promote overcharging, the zeta potential and the SFG measurements follow a similar trend; however, SHG yields the opposite behavior. The results can only be rationalized by considering a significant pH-dependent increase in the silica contribution. Using a simplistic, yet physically motivated model, we demonstrate that silica can interfere either constructively or destructively with water. By computing the hyperpolarizabilities of neutral and deprotonated silica clusters with density functional theory [CAM-B3LYP/6-31+G­(d,p)], we reveal that one potential source of this pH-dependent response of silica is a change in the hyperpolarizability upon the deprotonation of surface sites, suggesting that SHG is directly sensitive to surface charging. The direct sensitivity of SHG to the surface charge density of the substrate suggests that SHG would be a powerful tool in studying other mineral oxides such as alumina and titania.

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Reactions of 50-electron furyne and thiophyne clusters Ru3(CO)7(μ-H)(μ3-η2-C4H2E){μ-P(C4H3E)2}(μ-dppm) (1, 2; E = O, S) with thiols, dithiols, and benzeneselenol leads to the oxidative addition of the E–H bonds followed by concomitant elimination of the alkyne (probably as the alkene) to afford a range of new thiolato and selenolato triruthenium complexes. Addition of PhSH or iPrSH in boiling benzene affords the 48-electron clusters Ru3(CO)5(μ-SR)2{μ-P(C4H3E)2}(μ-dppm)(μ-H) (3–6; E = O, S; R = Ph, iPr) resulting from the addition of 2 equiv of thiol. In contrast, analogous reactions with 1,2-ethanedithiol and 1,3-propanedithiol yield the 50-electron clusters Ru3(CO)3{μ-S(CH2) n S)2{μ-P(C4H3E)2}(μ-dppm)(μ-H) (7–10; E = O, S; n = 2, 3), in which four S–H bonds have been activated. A similar multiple addition reaction is seen upon addition of PhSeH to 1, affording the tetraselenolato complexes Ru3(CO)4(κ1-SePh)(μ-SePh)3{μ-P(C4H3O)2}(μ-dppm)(μ-H) (11) and Ru3(CO)3(μ-SePh)4{μ-P(C4H3O)2}(μ-dppm)(μ-H) (12). Reaction of 2 with PhSeH gave the tetraselenolato complex Ru3(CO)4(κ1-SePh)(μ-SePh)3{μ-P(C4H3S)2}(μ-dppm)(μ-H) (13) together with bis(seleno)-capped Ru3(CO)5{PPh(C4H3S)2}(μ3-Se)2(μ-SePh)2(μ-dppm) (14) resulting from further cleavage of two selenium–carbon bonds and formation of a new carbon–phosphorus bond. The new clusters have been characterized by a combination of analytical and spectroscopic methods, and the molecular structures of 3, 4, 7, 8, and 11 have been determined by single-crystal X-ray diffraction studies. Complexes 7–10 are examples of 50-electron clusters containing three apparent metal–metal bonds; however, DFT calculations carried out for 7 show that the longest metal–metal interaction of 3.119 Å is actually held in place by the bridging thiolato and diphosphine ligands and does not represent a direct metal–metal bonding interaction.