Spore Detection in Air and Fluid Using Micro-cantilever Sensors
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Spore Detection in Air and Fluid Using Micro-cantilever Sensors Angelica P. Davila1,2, Amit Gupta1,2, Tom Walter3, Demir Akin1,2, Arthur Aronson3 and Rashid Bashir1, 2, 4 1
Birck Nanotechnology Center and Bindley Biosciences Center, School of Electrical and Computer Engineering, 3 Department of Biological Sciences, 4 Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Indiana, 47907 [email protected] 2
ABSTRACT The purpose of this paper is to report on our work to develop a real-time monitoring device by using micro-cantilevers for the mass detection of biological organisms in air and fluid. The biological agent used was Bacillus anthracis Sterne spore. The experiment was conducted using a laser Doppler vibrometer (LDV) to measure the resonant frequency of the thermal noise cantilevers and the corresponding decrease in frequency as a result of the added mass. Moreover, the added mass attributed to the spores was quantified and compared in air and deionized (DI) water. The silicon cantilevers used in this study were of lengths ranging from 25 µm to 50 µm, 200 nm thick and a width of approximately 9 µm. The first part of the experiment consisted of suspending spores onto the cantilevers in fluid, drying the cantilevers, performing measurements in air and extracting the mass of the added spores. The average mass of a spore in air was 367 fg. The second part of the experiment utilized antibody and bovine serum albumin (BSA) physically adsorbed onto the cantilevers in order to fix the spores on the surface during the measurements in deionized water. The extracted mass of a spore in fluid was measured to be an average of 1.85 pg. This study demonstrated the ability to detect biological samples not only in air but also in a liquid environment. INTRODUCTION Cantilevers have been used extensively to study and extract masses on the order of femotgrams and even attograms [1]. Different types of biological organisms have been detected, for instance, bacteria and viruses, however these detections have been mainly performed in air [2,3,4]. A cantilever that is used as a mass sensor functions by exhibiting a change in the natural frequency after the mass has been added to the cantilever. Once a particle is placed onto the cantilever the effective mass of the cantilever increases and the resonant frequency of the cantilever decreases. The change in frequency results from a change in mass and can be expressed as, ∆m =
k ⎛ 1 1 ⎞ − 2⎟ 2 ⎜ 2 4nπ ⎝ f1 f0 ⎠
(1)
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where k is the spring constant, n = 1 when the mass is place at the free end or n = 0.24 if it is a uniformly distributed mass on the cantilever, f1 is the loaded resonant frequency, the resonant frequency with the added mass, and f0 is the unloaded resonant frequency of the cantilever [2]. For the particles that are not placed at the free end, they represent an effective number of cells. The effective number of spores neff is determined by neff = nsp (l/lc), where nsp is the number of spores at a specific location, l is
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