Biosensors and Bioelectronics 22 (2007) 2065–2070 Towards the silicon nanowire-based sensor for intracellular biochemical detection Inkyu Park a,b,c,∗ , Zhiyong Li c,∗∗ , Xuema Li c , Albert P. Pisano a,b , R. Stanley Williams c a Berkeley Sensor and Actuator Center (BSAC), University of California, Berkeley, CA 94720-1774, United States b Mechanical Engineering Department, University of California, Berkeley, CA 94720-1740, United States c Quantum Science Research, Hewlett-Packard Laboratories, 1501 Page Mill Road, MS 1123, Palo Alto, CA 94304, United States Received 24 April 2006; received in revised form 5 August 2006; accepted 7 September 2006 Available online 23 October 2006 Abstract A microneedle sensor platform with integrated silicon nanowire tip was developed for intracellular biochemical detection. Because of the virtue of miniaturized size and high sensitivity, this sensor has a great potential for studying individual cell or localized bioenvironment by revealing the pH level and/or enzyme activities. The fabrication of the microneedle sensor was primarily based on conventional silicon processing, where a silicon-on-insulator (SOI) wafer with 50 nm thick (1 0 0) p-type Si device layer was used as the substrate. The silicon nanowires of 50 nm height and 50–100 nm width were created by electron beam (E-beam) lithography on the tip of microneedle with good electrical connection to the contact pads for convenient electrical measurement. A three layer structure with base, support cantilever, and needle tip was designed to ensure convenient handling of sensors and minimize the invasive penetration into biological cells. In this paper, we demonstrate a preliminary assessment of this novel intracellular sensor with electrical conductance measurement under different pH levels. It is expected that this sensor with proper chemical modification will enable localized biochemical sensing within biological cells, such as neurotransmitter activities during the synaptic communication between neuron cells. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon nanowire; In vitro detection; pH sensor; Protein detection 1. Introduction At present, the study of physiological activities of live organ- isms in the individual cellular or organelle level is drawing a great attention in various communities such as biomedical science, molecular and cell biology, pharmacology, etc. Rep- resentative examples include the monitoring of metabolic pro- cesses such as protein synthesis or secretion from individual cells and the detection of neurotransmitters used in communications between nerve cells. Thus far, a number of different approaches have been introduced in the single-cell studies. Fluorescence microscopy has been widely used to detect pH levels (Robey et al., 1998; Olsen et al., 2002), oxygen levels (Ji et al., 2002; Hynes et al., 2003), and ATP contents (Iijima et al., 2003; Kubitscheck ∗ Corresponding author. Tel.: +1 510 642 8713. ∗∗ Corresponding author. E-mail addresses: inkyu@eecs.berkeley.edu, inkyupark.ucb@gmail.com (I. Park), zhiyong.li@hp.com (Z. Li). et al., 1995) within individual animal cells. However, these opti- cal methods based upon fluorescence can introduce side effects such as interaction between luminescent agents and biochemical molecules in the metabolic processes. Also, the attenuation of optical signals due to photo-bleaching does not allow long-term monitoring of cells. Alternatives to this optical method are electrochemical sens- ing by microelectrodes and solid-state device sensing by chem- ical field effect transistor (ChemFET). Fast-scan cyclic voltam- metry (FSCV) with carbon fiber microelectrodes in micron-sized dimensions have been used in the detection of neurotransmit- ters such as histamine from mast cells (Pihel et al., 1995), dopamine from rat striatum (Bergstrom and Garris, 1999), and catecholamine from chromaffin cells (Wightman et al., 1991). Ion-sensitive field effect transistor (ISFET), a type of Chem- FET, has been used to measure the pH level changes by cell metabolism (Baumann et al., 1999; Lehmann et al., 2001). In this paper, we propose a novel in vitro single cell sensor, which is a microneedle sensor with integrated silicon nanowire sensing components for biochemical detection in intracellular 0956-5663/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2006.09.017