Letter pubs.acs.org/Langmuir
Nanodimensionally Driven Analyte Response Reversal in Gold Nanocluster Chemiresistor Sensing Arthur W. Snow,* Mario G. Ancona, and Doewon Park Naval Research Laboratory, Washington, D.C. 20375, United States S Supporting Information *
ABSTRACT: The modulation of electron transport through an ensemble of ligand-stabilized gold nanoclusters by the sorption of vapors is made exceptionally sensitive and selective by terminal carboxylic acid functionalization of the alkanethiol ligand. Of further importance, the directionality of the response (conductance increase or decrease) is strongly dependent on the nanoscale dimensions of the gold core and ligand shell thickness. Films of gold nanoclusters composed of a 2 nm metal core with a 0.5 nm −S(CH2)5COOH shell are compared to those based on an 8 nm core and a 1.5 nm −S(CH2)15COOH shell with the finding of very strong and selective responses to amine vapors but with a reversal of response in the direction of the conductance transduction. This unexpected result cannot be accommodated by known vapor response transduction mechanisms based on a swelling expansion and a dielectric alteration of the ligand shell to modulate conductance in the ensemble. A speculative new mechanism is proposed on the basis of intercluster nanodomains of low and high dielectric character whose domain dimensions are determined by the ligand molecular structure and dielectric character that can range up to that associated with an ionic capacitance if generated by a vapor interaction.
■
■
INTRODUCTION The concept of a chemiresistor vapor sensor based on a thin transductive film of ligand-stabilized gold nanoclusters was introduced in 1998, and such sensors were referred to as MIME (metal−insulator−metal ensemble) sensors.1,2 At that time, it was observed that the direction and magnitude of the electrical response varied depending on the particular analyte and the gold cluster surface ligands involved. This directional dependence of the sensor’s electrical response (i.e., a conductance increase vs decrease) was a feature of particular interest with respect to developing sensor selectivity. Since then, considerable research by many investigators has been conducted on this specific type of chemiresistor as reflected by published reviews of these efforts over the previous decade.3−10 With respect to the sensor response transduction mechanism, these reviews and several references cited therein (particularly refs 11−18) comment on this vapor response bidirectionality and correlate it with either a ligand shell swelling effect for the generally observed conductance decrease or a dielectric effect when a conductance increase is observed. In this letter, we report an unexpected observation of an exceptionally strong bidirectionality in vapor response that is dependent not on the identity of the vapor nor on different functional groups in the ligand shell but instead on the relative nanoscale dimensions of the cluster core and ligand shell. This work was undertaken with the objective of developing point sensor systems with enhanced sensitivity and selectivity for targeted vapors, and the approach focuses on an amine−carboxylic acid interaction. © 2012 American Chemical Society
EXPERIMENTAL SECTION
Materials. The Au(2nm):C5COOH cluster was prepared by a ligand exchange reaction of HS(CH2)5COOH with a hexanethiol ligand-stabilized 2 nm gold cluster, and the Au(8nm):C15COOH cluster was prepared by a ligand exchange reaction of HS(CH2)15COOH with a tetraoctylammonium bromide-stabilized 8 nm gold cluster. Complete details of the synthesis and purification are in the Supporting Information. Devices. MIME sensor devices were fabricated by the airbrush deposition of solutions of the Au(2nm):C 5COOH and Au(8nm):C15COOH clusters onto 100 °C heated, 15 μm interdigital electrodes (Microsensor Systems, Inc. P/N 302) in a manner similar to that described previously1,2 and in more detail in the Supporting Information. Measurements. MIME sensor vapor response measurements were conducted with devices housed in a low-dead-volume custom-made cell with vapors delivered from 15 °C bubblers at zero air diluted concentrations ranging from P/Po = 2 to 10−5 computer controlled by a vapor generator system (Microsensor Systems, Inc., model VG400). Additional detail is provided in the Supporting Information.
■
RESULTS AND DISCUSSION The Au nanocluster system, the vapor interaction, and a representative sensor response are depicted in Figure 1. As shown, triethylamine vapor interacts with a terminal carboxylic acid functional group of a ligand shell molecule to form an Received: August 15, 2012 Revised: October 22, 2012 Published: October 23, 2012 15438
dx.doi.org/10.1021/la303319j | Langmuir 2012, 28, 15438−15443
Langmuir
Letter
Figure 1. (a) Depiction of triethylamine vapor interacting with −COOH functional group in a HS(CH2)5COOH-stabilized gold cluster to form the triethylammonium carboxylate complex and the necessary heating to reverse the process completely. (b) Corresponding Au(2nm):C5COOH sensor response (current vs time) for initial and repeated exposures to triethylamine vapor (P/Po = 0.01, 468 ppm) and after total regeneration (80 °C/10 min).
shape of the triethylamine isotherm indicates saturation at P/Po > 10−3. The surprising behavior that motivates this letter arises when the cluster core diameter and shell thickness are changed. The vapor response data described above were obtained from a MIME sensor that used clusters (designated as Au(2nm):C5COOH) with an average core diameter of 2 nm and an −S(CH2)5COOH shell thickness of approximately 0.5 nm. When a chemically nearly identical sensor is fabricated with enlarged clusters (designated as Au(8nm):C15COOH) having an average core diameter of 8 nm and a −S(CH2)15COOH shell thickness of approximately 1.5 nm, the responses (Figure 2c,d) are found to be quite similar in sensitivity and selectivity as before, but the polarity of the response is reversed for all vapors tested. Again, we emphasize that the chemical natures of the vapors and the carboxylate-terminated alkanethiol clusters have not been changed and that only the numbers of gold atoms and methylene units have been adjusted, resulting in differing core/shell dimensions. Thus, the origin of the polarityreversal phenomenon appears to reside in the nanoscale geometry of the cluster itself, and the remainder of this letter is devoted to discussing this intriguing finding. As noted above, the MIME sensor responds to vapor exposure by a perturbation in the conductance of the gold
ammonium carboxylate complex. The reversible formation of this complex is easily demonstrated by the appearance and disappearance of specific bands in the infrared spectrum that accompany the sorption and desorption of triethylamine vapor on and off a film of stearic acid (Figures S1 and S3 and discussion in the Supporting Information). Although this ammonium carboxylate chemistry is complicated by multiple stoichiometries,19−21 the sensor’s response to a vapor exposure−purge cycle displays very strong, rapid, and partially reversible responses to the vapor exposures that are also presented in Figure 1b. These responses are in the direction of decreasing conductance with the initial response being 2 to 3 times larger than that to succeeding exposures. That a brief (
