Development of a Screening Platform for Optimizing Chemical Nanosensor Materials

Abstract

Chemical sensors, relying on changes in the electrical conductance of a gas-sensitive material due to the surrounding gas, typically react with multiple target gases and the resulting response is not specific for a certain analyte species. The purpose of this study was the development of a multi-sensor platform for systematic screening of gas-sensitive nanomaterials. We have developed a specific Si-based platform chip, which integrates a total of 16 sensor structures. Along with a newly developed measurement setup, this multi-sensor platform enables simultaneous performance characterization of up to 16 different sensor materials in parallel in an automated gas measurement setup. In this study, we chose the well-established ultrathin SnO₂ films as base material. In order to screen the sensor performance towards type and areal density of nanoparticles on the SnO₂ films, the films are functionalized by ESJET printing Au-, NiPt-, and Pd-nanoparticle solutions with five different concentrations. The functionalized sensors have been tested toward the target gases: carbon monoxide and a specific hydrogen carbon gas mixture of acetylene, ethane, ethne, and propene. The measurements have been performed in three different humidity conditions (25%, 50% and 75% r.h.). We have found that all investigated types of NPs (except Pd) increase the responses of the sensors towards CO and HC_mix and reach a maximum for an NP type specific concentration.

Keywords: hybrid nanomaterials; metal oxide gas sensors; nanoparticles; nanosensors.

Figure 1

2 × 2 cm² sized Si-based platform chip integrating 16 sensor devices. The chip enables conductive sensor measurement in a 4-point measurement configuration and exhibits 64 contact pads for the gas sensors; 2 additional pads are required for the Pt100-like temperature sensor. The red rectangle designates the “hot area” heated by the Au-coated Cu-block underneath. The insert shows a final processed 100 × 50 µm² sized SnO₂ film; the active sensor area between the electrodes measures 30 × 50 µm².

Figure 2

(a) Measurement setup (“open state”) consisting of a holder for the platform chip and a lid with gas inlet, electrical connectors, and contact pins. (b): Front view of the lid, which holds two prober heads with a total of 66 contact pins. (c) The platform chip is placed on a massive heater block (fabricated with copper, coated with gold) which heats the central part of the chip from underneath up to 350 °C. As soon as the lid is closed, the pins contact 66 Ti/Pt pads on the platform chip and enable simultaneous characterization of all 16 sensor structures in parallel.

Figure 3

Left: Schematic of an ESJET system. A capillary (“emitter”) with the printing ink and a needle electrode is positioned a short distance above the grounded substrate. Right: ESJET nozzle positioned above a single SnO₂ sensor film on the Si-platform chip; the NP-dot diameter matches the active area of the sensor films, which measures 30 × 50 µm².

Figure 4

Printing scheme of the NP solutions with different NP concentrations (1:1, 1:2, 1:4, 1:8, 1:16) on the SnO₂ sensor films on the Si-platform chips. Two sensors, respectively, are functionalized with the same NP concentration; six SnO₂ sensor films are not functionalized and form the reference sensors.

Figure 5

SEM graph of Sensor A, which is functionalized with Au-NPs; the Au-NPs can be clearly seen as white dots.

Figure 6

Transmission electron microscopy images of the noble metal NP batches used for sensor functionalization. Scale bars: 200 nm (left column), and 50 nm (right column). All particle batches showed approximately spherical particle shapes with average diameters of 5.3 ± 0.5 nm, 4.4 ± 0.5 nm, and 3.3 ± 0.4 nm for the Au, Pd, and NiPt material systems, respectively.

Figure 7

Typical resistance behavior of bare and an Au-NP functionalized SnO₂ sensors (black and red curves, respectively) during exposure to 50 ppm of CO and HC_mix at 300 °C operation temperature (bottom red) at 50%, 25% and 75% humidity levels (bottom black).

Figure 8

Response of bare SnO₂ sensors and SnO₂ sensors functionalized with different concentrations of Au-NP inks towards (a) 50 ppm CO, and (b) 50 ppm HC_mix at 300 °C operation temperature.

Figure 9

Response of bare SnO₂ sensors and SnO₂ sensors functionalized with different concentrations of NiPt-NP inks towards (a) 50 ppm CO, and (b) 50 ppm HC_mix at 300 °C operation temperature.

Figure 10

Response of bare SnO₂ sensors and SnO₂ sensors functionalized with different concentrations of Pd-NP inks towards (a) 50 ppm CO, and (b) 50 ppm HC_mix at 300 °C operation temperature.

Figure 11

Response of sensors functionalized with Au, Pd, and NiPt-NPs with different concentrations towards (a) 50 ppm CO, and (b) 50 ppm HC_mix at 50% rh and 300 °C operation temperature normalized to the response of bare SnO₂ sensors.

Figure 12

Proposed CMOS-integrated multi-sensor device. By using proper combinations of MOx film, and types of NPs on the 8 µhps (exemplarily shown for 4 µhps), we will realize a 5 × 5 mm² sized multi-gas sensor device capable of simultaneous detection of several target gases.