Publications

Abstract not provided.

Abstract not provided.

Molecular electronic based chemical vapor sensors were assembled using noble metal nanoparticles and short conjugated phenylene ethynylene (PE) based molecules. Sacrificial capping ligands on the nanoparticles were replaced by tighter binding PE ligands. The films were assembled between pairs of electrodes by iteratively exposing the substrates to solutions of the nanoparticles and PE crosslinking bridging ligands. Some of the conjugated bridging molecules contained an electron deficient phenol to provide a simple platform for developing sensor applications. The phenol is calculated to have a significant change in its HOMO/LUMO gap in the presence of specific analytes. Judicious combination of nanoparticle size and ligand structure provides a film in which the organic bridging ligands dramatically affect film conductance. Specifically, {pi}-conjugated ligands lower resistance more in films with smaller particles. Thus the sensing mechanism of these films is not based on the typical swelling mechanism but rather on the modulation of the molecular electronic structure of the conducting PE bridging ligands. Interdigitated Au electrodes built on quartz substrates were first silanized with tetrakis(dimethylamino)silane. The remaining amino functionalities were displaced with 1,8-octanedithiol (ODT) to give a thiolated surface capable of binding nanoparticles. The substrate was then incubated in a solution of dodecylamine-capped nanoparticles. The film thickness was increased via alternating exposure to solutions of bifunctional crosslinking molecules and nanoparticles (Figure 1). Nanoparticles and assembled films were characterized by TEM and AFM prior to electrical characterization. After verifying the selectivity of this new attachment chemistry, a novel robotic sample preparation was employed to build nanoparticle films of different thickness on prepared electrodes. By preparing the nanoparticle films using a robot, many problems with irregularities of the deposited films were eliminated. This sample preparation system was designed with the capability to measure the resistivity of the nanoparticle films after assembly of each layer. Using such a sample preparation system is vital for developing mass-produced sensors from nanoparticle films. The robotic system was used to deposit and measure the electrical properties of Pt and Au nanoparticles linked with different ligands such as ODT and meta-PE diisocyanide. Figure 2 is a plot showing the resistance vs. film layer for several combinations of nanoparticles and linker-ligands. The data shows that the resistance of the film drops and eventually saturates as additional nanoparticle layers are deposited. There is also an inversion in the resistance per layer that depends on the nanoparticle's type and the ligand used to crosslink the film. This data is significant because it shows how the selection of certain nanoparticle properties (such as size and material) and selection of an appropriate linking ligand can be used to tune the conductance of a film composed of nanoparticles. It is well known that smaller nanoparticles have a higher charging potential. This coupled with the inherent variability of organic molecules ensures that a film in which the organic molecules dominate conductivity can be achieved. In addition to the experiments above, nanoparticle films were assembled using cross-linkers that can be modified by an analyte. Figure 3 shows a typical I(V) curve for a Au nanoparticle film crosslinked with a phenylene ethynylene based electron deficient phenol. There is a clear reversible change in the resistance of the film when exposed first to acid and then base. The generation of a new response mechanism for nanoparticle films greatly increases the scope of organic/nanoparticle films for sensor applications. Their crosslinked nature increases their robustness and allows for use in both aqueous as well as organic solutions. In summary, we have developed a novel reproducible sample preparation system for the deposition of crosslinked nanoparticle films on a variety of substrates. This system has the ability to acquire electrical data during the sample deposition. Data collected for several nanoparticle film depositions demonstrated the ability to tune the conduction of the film by the selection of nanoparticle size and the cross-linking ligand. The material we have developed is a hybrid intermediate between a true organic conducting polymer and a classical nanoparticle film. The nanoparticles provide a scaffold on which to assemble various conducting/sensing oligomers and ligands without the problems inherent to conducting polymers.

Abstract not provided.

Nanoparticles have received much attention and have been the subject of many reviews. Nanoparticles have also been used to form super molecular structures for molecular electronic, and sensor applications. However, many limitations exist when using nanoparticles, including the ability to manipulate the particles post synthesis. Current methods to prepare nanoparticles employ functionalities like thiols, amines, phosphines, isocyanides, or a citrate as the metal capping agent. While these capping agents prevent agglomeration or precipitation of the particles, most are difficult to displace or impede packing in nanoparticle films due to coulombic repulsion. It is in this vein that we undertook the synthesis of nanoparticles that have a weakly bound capping agent that is strong enough to prevent agglomeration and in the case of the platinum particles allow for purification, but yet, easily displaced by other strongly binding ligands. The nanoparticles where synthesized according to the Brust method except stearonitrile was used instead of an aliphatic thiol. Both platinum and gold were examined in this manner. A representative procedure for the synthesis of platinum nanoparticles involved the phase transfer of chloroplatinic acid (0.37 g, 0.90 mmol) dissolved in water (30 mL) to a solution of tetraoctylammonium bromide (2.2 g, 4.0 mmol) in toluene (80 mL). After the chloroplatinic acid was transferred into the organic phase the aqueous phase was removed. Stearonitrile (0.23 g, 0.87 mmol) was added and sodium borohydride (0.38 g, 49 mmol) in water (25 mL) was added. The solution turned black almost immediately and after 15 min the organic phase was separated and passed through a 0.45 {micro}m Teflon filter. The resulting solution was concentrated and twice precipitated into ethanol ({approx}200 mL) to yield 0.11 g of black platinum nanoparticles. TGA experiments showed that the Pt particles contained 35% by mass stearonitrile. TEM images showed an average particle size of 1.3 {+-} 0.3 nm. A representative procedure for the synthesis of gold nanoparticles involved the transfer of hydrogen tetrachloroaurate (0.18 g, 0.53 mmol) dissolved in water (15 mL) to a solution of tetraoctylammonium bromide (1.1 g, 2.0 mmol) in toluene (40 mL). After the gold salt transferred into the organic phase the aqueous phase was removed. Stearonitrile (0.23 g, 0.87 mmol) was added and sodium borohydride (0.19 g, 5.0 mmol) in water (13 mL) was added. The solution turned dark red almost immediately, and after 15 min the organic phase was separated and passed through a 0.45 {micro}m Teflon filter. The resulting solution was used without purification via precipitation because attempts at precipitation with ethanol resulted in agglomeration. TEM images showed an average particle size of 5.3 {+-} 1.3 nm. The nanoparticles synthesized were also characterized using atomic force microscopy in tapping mode. The AFM images agree with the TEM images and show a relatively monodispersed collection of nanoparticles. Platinum nanoparticles were synthesized without stearonitrile to show that the particles were in fact capped with the stearonitrile and not the tetraoctylammonium bromide. In the absence of stearonitrile the nanoparticles would not redissolve in hexane or toluene after precipitation. While it is possible the tetraoctylammonium bromide helps prevent agglomeration by solvation into the capping stearonitrile ligand layer on the particles recovery of a quantitative amount of the starting tetraoctylammonium bromide was difficult and we cannot rule out that some small amount of tetraoctylammonium bromide serves in a synergistic capacity to help solubilize the isolated platinum particles. Several exchange reactions were carried out using the isolated Pt nanoparticles. The stearonitrile cap was exchanged for hexadecylmercaptan, octanethiol, and benzeneethylthiol. In a typical exchange reaction, Pt nanoparticles (10 mg) were suspended in hexane (10 mL) and the exchange ligand was added (50 {micro}L). The solutions were allowed to stir overnight and precipitated twice using ethanol. TGA experiments confirmed ligand exchange. We have also shown that these particles may be assembled in a layer by layer (LBL) fashion to build up three dimensional assemblies. As an example of this LBL assembly a substrate consisting of gold electrodes separated by 8 {micro}m on a quartz wafer was first functionalized by immersing in a solution of 1,8-octanedithiol (50 {micro}L) in hexane (10 mL) for 15 min, rinsed with hexane (10 mL), ethanol (10 mL), and dried under a stream of nitrogen. The scaffold was then placed in a toluene solution containing Au nanoparticles capped with stearonitrile (10 mg/mL) for 15 minutes. The scaffold was then rinsed with hexane (10 mL), ethanol (10 mL), and dried under a stream of nitrogen. The substrate was then immersed iteratively between the 1,8-octanedithiol and the Au nanoparticle solution 4 more times.