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Abstract | Adsorption | Sensor

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Development of a MEMS Sensor based on arrays of Silicon micro cantilevers Manesh Zachariah1, Paolo Bonanno1, Marina Dipasquale1, Roberto Raiteri 1* 1 Department of Biophysical and Electronic Engineering, University of Genova, Italy rr@unige.it Abstract Microcantilever based sensors are highly flexible and sensitive devices. By coating the cantilever surface with a material that selectively adsorbs or binds a target substance, a cantilever can be converted into a highly selective chemical and
  1 Development of a MEMS Sensor based on arrays of Silicon micro cantilevers Manesh Zachariah 1 , Paolo Bonanno 1 , Marina Dipasquale 1 , Roberto Raiteri 1* 1 Department of Biophysical and Electronic Engineering, University of Genova, Italyrr@unige.it   Abstract Microcantilever based sensors are highly flexible and sensitive devices. By coating the cantilever  surface with a material that selectively adsorbs or binds a target substance, a cantilever can beconverted into a highly selective chemical and biochemical sensor. Cantilevers can transform achemical reaction into a mechanical motion, either a slow bending or a change in the oscillationbehavior, which can be measured directly and with nanometer resolution with an optical-lever based technique. We observed that the adsorption of thioled carboxylic acid on the gold coated surface of a silicon microfabricated cantilever produces a static bending of the cantilever due to a change of its surface stress. The phenomena of surface stress were investigated relative to the formation of self-assembled monolayers on the surface of the cantilever, resulting in the functionalization of the gold  surface.  Keywords  — microcantilevers, MEMS, sensor array  1.INTRODUCTION Silicon microcantilevers have been employed for physical, chemical, and biological sensing [1, 2].Microcantilever based sensors (MCSs) have been proposed in different fields, ranging from clinicaldiagnostics to the detection of chemical and biological warfare agents. These sensors show several potential advantages over conventional analytical techniques in terms of high sensitivity, low cost,simple operation, small volume of analyte required, and fast response. Molecules adsorbed onto amicrocantilever can cause resonance frequency shifts, due to the added mass, and/or changes in thesurface stress which induces static deflections of the whole cantilever. The surface stress transduction principle is based on adsorbate-substrate and adsorbate-adsorbate interactions, independently from themass of the adsorbate. This makes the surface stress transduction principle very attractive for thedetection of “light” (i.e. small) molecules. In order to develop a sensor platform based onmicrocantilever arrays as surface stress transducers we studied how surface stress variations induced  2  by molecular absorption are affected by physical and chemical modification of the cantilever surfaceand the presence of gold nanoparticles adsorbed on the surface. 1.2 METHODS  We used an array of micro-fabricated rectangular cantilevers, 500 µm long, 150 µm wide, 1 µm thick,made of single crystal Si (100). Because of the low spring constant (k ≈ 0.05 N/m) and the highfundamental resonance frequency (f  0 ≈ 4.8 KHz in air) they are both highly sensitive to applied forcesand broadly immune to external mechanical noise. The experimental set up is based on an array of 20micro cantilevers divided in 4 identical groups (labeled as “Well” A, B, C and D in Fig. 2), each wellis compound by 4 cantilevers plus1 reference mirror. Cantilever deflections were monitored using adevice presented earlier [3], that utilizes an optical beam deflection readout with a linear array of vertical cavity surface emitting lasers (VCSELs) with wavelength of 760 nm, and an array of microfocusing lenses. The laser spots which are switched on sequentially, one at a time, and focusedonto the cantilever free ends using an array of microfocusing lenses, are then reflected by the goldsurface to a linear PSD. The laser power can be adjusted individually for each VCSEL to obtain adesired intensity signal at the PSD. Fig.1 (a) schematic drawing of a cantilever coated on one side with a receptor layer ( left  ). Bending due to a change in surfacestress induced by adsorption of molecules on the receptor layer ( right  ). (b) schematic drawing of an oscillating cantilever withan added mass at its free end ( left  ) and cantilever resonance frequency shift caused by the added mass ( right  ) (b)(a)  3 Fig 2(a ). A sketch of the optical laser beam deflection readout system of the MCS array. Shown are thefour MCSs and the fixed mirror within the four wells A, B, C, and D respectively. (  b ). Microcantilever  bending is induced by the interactions   of probe molecules with the sensor coating of MCS.(c).Scanning electron microscope ( SEM ) micrograph of one well within the MCS array.To deliver the carrier fluid and different sample solutions to the cantilever array, a syringe pumpfluidic system it’s used. Each well is connected to one syringe, all four syringes can then be operatedin parallel. The system consists of four valves for the carrier fluid, four syringes, four sample valveloops, four injection valves for sample solution, and one cartridge containing the fluid cell with thefour wells.Fig 3 A schematic outline of the fluidic delivery system for the MCS array.The following protocol was used for the functionalization of the cantilvers chip with goldnanoparticles.At first, the silicon array, coated on one side with gold, was cleaned using RCA method to remove allorganic and metallic contaminants from well A, B and C leaving the srcinal sputtered goald on wellD. After the cleaning process, it was silanized in vapour phase with amino propyl triethoxy  4 silane(APTES) molecules. Then we immersed well A, B and C of the silanized chip in a 13nm goldnanoparticles citrate solution, so that well D was not functionalized with gold nanoparticles. Negativecharged 13nm gold nanoparticles were adsorbed on to the positive charged amino groups of amino propyl triethoxy silane (APTES) of the silicon substrate by immersion of well A, B & C of thesilanized silicon chip into a 4.5×10 8 M citrate solution of gold colloid for 24hour. After that chip wasmounted on the cartridge and was immersed in milli.Q water at a constant flow rate of 5l/min untilequilibrium in the drift of the cantilever deflection was achieved. The 200 l 12-mercaptododecanoicacid solutions were then injected in to the measurement chamber, followed by milli.Q water again toremove unbound acid molecules from the cantilever surfaces. 1.3 RESULTS AND DISCUSSION  Using a constant flow (flow rate of 5l/min) of Milli-Q water as carrier we injected 200 l of 12-mercaptododecanoic acid solutions in order to create a thiol monolayer on the colloidal( well A,B andC) and sputtered (well D) gold layer while recording the cantilevers deflection. Figure 4 shows theresult obtained for the wells A, B and C of a silanized chip functionalised with gold nanoparticles,while D of the same chip without gold nanoparticles. Deflection response up on adsorption of 12-mercaptododecanoic monolayer is well visible.. The12-mercaptododecanoic acid was successively bound to the gold nanoparticles and on the sputtered gold by a sulfur-gold bond.While the deflection peak (the minimum around min. 300 in Fig.4) it’s comparable in magnitude for all the well, it’s well visible that after the rinsing there is net change in the deflection for well D,almost double respect the others wells. This noticeable change in well D means that in well A, B and Cwhere the cantilevers are coated with gold nanoparticles the so deposited monolyer have an highdensity compared to the one in well D.In conclusion we have seen that the MCSs are suitable sensors for monitoring the formation of monolayers. In addition the use of gold nanoparticles as a colloidal layer seems to be a promisingtechnique to promote the deposition of a high density monolayer and in general to enhance thesensibility of this particular kind of sensors. Further investigation must be performed to check thereproducibility of this technique.A long term goal of this research activity is to develop sensors for homeland security, which: 1) candetect multiple threats simultaneously and rapidly; 2) are inexpensive, miniaturized, and robust so thatcan be deployed almost anywhere (e.g. airports, seaports, public buildings, strategic locations) and 3)have built-in telemetry for data transmission and networking.
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