The influence of counter-ion adsorption on the ψ0/pH characteristics of insulator surfaces

The site-binding theory of Yates, Levine, and Healy is extended to include the possibility that counter-ion binding of anions and cations occurs at different distances from the insulator surface. A method for straightforward computation of the ψ0/σ0/pH characteristics is given. This theory is applied to the study of electrolyte/insulator/silicon structures, which makes it possible to measure the ψ0/pH characteristics. Measurements are presented for structures where the insulator is γ-Al2O3 deposited by chemical vapour deposition at 900°C. The influence of counter-ion binding on the ψ0/pH curves is a second-order effect compared to the site-dissociation acid/base reactions, but it is clearly visible. Consideration of the influence of the ionic strength of the electrolyte leads to an estimated anion adsorption equilibrium constant in the range of 0.05 to 0.4 mol−1 dm3 in chloride solutions, although no significant influence of the type of ions present could be observed. Application of the theory to existing measurements of the ψ0/pH and σ0/pH curves of SiO2 surfaces indicates that for this material the cation adsorption equilibrium constant is in the order of 0.1 mol−1 dm3

Spatially Controlled Membrane Depositions for Silicon-Based Sensors

The membrane deposition technology on silicon-based transducers constitutes the most delicate part of the miniaturized (bio)chemical sensor fabrication. Membrane adhesion to the transducer, reproducibility of the deposition process and its spatial control are the three most important parameters which determine the sensor performance and lifetime.The fabrication of two sensors is described: 1) a combined pO2, pCO2, pH sensor for which a polyacrylamide gel and a polysiloxane gas-permeable membrane were deposited and patterned at the on-wafer level and 2) a glucose amperometric enzyme electrode where the glucose oxidase was immobilized electrochemically either in a polypyrrole matrix or co-deposited with bovine serum albumin by electrochemically aided adsorption. The optimization of the deposition procedures allowed reproducible devices with reasonable lifetimes to be obtained

Low Fluorescence Enzyme Matrices Based on Microfabricated SU-8 Films for a Phenol Micro-Biosensor Application

In this contribution, the possibility of using SU-8 photoresist, a polymer widely used in MEMS applications, for the development of inexpensive and disposable optical phenol micro-biosensors is explored. The immobilisation of the enzyme, the encapsulation of the indicator and the patterning of the SU-8 were accomplished simultaneously in a simple one step microfabrication process. The enzyme still showed activity after encapsulation in SU-8 although the process involved its embedding in a hard and rigid epoxy resin matrix. This was carried out by measuring the signal of an oxygen-sensitive indicator (ruthenium-complex) through monitoring of the enzymatic oxidation of phenol which consumes oxygen. Films without enzyme showed negligible variation in fluorescence intensity upon phenol addition, whereas films with encapsulated enzyme and oxygen-sensitive fluorescent indicators showed a very clear change in fluorescence intensity upon addition of phenol. The current work demonstrates a new concept of a low cost immobilisation technique in combination with the microfabrication process for biosensor technology

Multisite monitoring of choline using biosensor microprobe arrays in combination with CMOS circuitry

A miniature device enabling parallel in vivo detection of the neurotransmitter choline in multiple brain regions of freely behaving rodents is presented. This is achieved by combining a biosensor microprobe array with a custom-developed CMOS chip. Each silicon microprobe comprises multiple platinum electrodes that are coated with an enzymatic membrane and a permselective layer for selective detection of choline. The biosensors, based on the principle of amperometric detection, exhibit a sensitivity of 157±35 µA mM-1 cm-2, a limit of detection of below 1 µM, and a response time in the range of 1 s. With on-chip digitalization and multiplexing, parallel recordings can be performed at a high signal-to-noise ratio with minimal space requirements and with substantial reduction of external signal interference. The layout of the integrated circuitry allows for versatile configuration of the current range and can, therefore, also be used for functionalization of the electrodes before use. The result is a compact, highly integrated system, very convenient for on-site measurement

Multisite monitoring of choline using biosensor microprobe arrays in combination with CMOS circuitry

A miniature device enabling parallel in vivo detection of the neurotransmitter choline in multiple brain regions of freely behaving rodents is presented. This is achieved by combining a biosensor microprobe array with a custom-developed CMOS chip. Each silicon microprobe comprises multiple platinum electrodes that are coated with an enzymatic membrane and a permselective layer for selective detection of choline. The biosensors, based on the principle of amperometric detection, exhibit a sensitivity of 157 +/- 35 mu A mM(-1) cm(-2), a limit of detection of below 1 mu M, and a response time in the range of 1 s. With on-chip digitalization and multiplexing, parallel recordings can be performed at a high signal-to-noise ratio with minimal space requirements and with substantial reduction of external signal interference. The layout of the integrated circuitry allows for versatile configuration of the current range and can, therefore, also be used for functionalization of the electrodes before use. The result is a compact, highly integrated system, very convenient for on-site measurements

Microactuators based on ion implanted dielectric electroactive polymer (EAP) membranes

We report on the first successfully microfabricated and tested ion-implanted dielectric electroactive polymer (EAP or DEAP) actuators. Dieletric EAP (DEAP) actuators combine exceptionally high energy-density with large amplitude displacements [1,2]. Scaling DEAPs down to the milimeter and micron scale requires patterning compliant electrodes on such a scale on the surfaces of the polymer. We used ion implantation to make the surfaces of the polymer locally conducting. Implanting the compliant electrodes solves the problem of microfabricating patterned electrodes whose elasticity is close to that of the insulating elastomer, thus avoiding the deposition of metal electrodes on the polymer which leads to significant stiffening of the membrane [3]. Several techniques based on ion implantation for chip level and wafer level fabrication are presented. Ion implanted DEAP membranes were both simulated (FEM) and characterized. We report measurements on an actuator consisting of a 30-um-thick ion implanted PDMS membrane bonded to a silicon chip into which a cavity had been etched. We measured 110 um vertical displacements for a 0.72 mm2 square membrane, achieving for the first time the same percent displacement in microscopic EAPs as in macroscopic devices. These observations show that ion implantation allows the patterning of electrodes on PDMS membranes with negligible increase in stiffness

Droplet-based Liquid-Liquid Extraction and On-Chip IR-Waveguide-Spectroscopy Detection of Cocaine in Human Saliva

We present a portable microsystem to quantitatively detect cocaine in human saliva. The chip combines a simple microfluidic method for multiphase liquid-liquid extraction to transfer cocaine from IR-light absorbing saliva to an IRtransparent solvent with the on-chip cocaine detection by IR-waveguide-spectroscopy (QC-laser, waveguide, detector). With our droplet-based extraction method we achieve an additional analyte pre-concentration of at least two orders of magnitude

Focal areas of increased lipid concentration on the coating of microbubbles during short tone-burst ultrasound insonification

Acoustic behavior of lipid-coated microbubbles has been widely studied, which has led to several numerical microbubble dynamics models that incorporate lipid coating behavior, such as buckling and rupture. In this study we investigated the relationship between micro-bubble acoustic and lipid coating behavior on a nanosecond scale by using fluorescently labeled lipids. It is hypothesized that a local increased concentration of lipids, appearing as a focal area of increased fluorescence intensity (hot spot) in the fluorescence image, is related to buckling and folding of the lipid layer thereby highly influencing the microbubble acoustic behavior. To test this hypothesis, the lipid microbubble coating was fluorescently labeled. The vibration of the microbubble (n= 177; 2.3-10.3 μm in diameter) upon insonification at an ultrasound frequency of 0.5 or 1 MHz at 25 or 50 kPa acoustic pressure was recorded with the UPMC Cam, an ultra-high-speed fluorescence camera, operated at ∼4-5 million frames per second. During short tone-burst excitation, hot spots on the microbubble coating occurred at relative vibration amplitudes > 0.3 irrespective of frequency and acoustic pressure. Around resonance, the majority of the microbubbles formed hot spots. When the microbubble also deflated acoustically, hot spot formation was likely irreversible. Although compression-only behavior (defined as substantially more microbubble compression than expansion) and subharmonic responses were observed in those microbubbles that formed hot spots, both phenomena were also found in microbubbles that did not form hot spots during insonification. In conclusion, this study reveals hot spot formation of the lipid monolayer in the microbubble's compression phase. However, our experimental results show that there is no direct relationship between hot spot formation of the lipid coating and microbubble acoustic behaviors such as compression-only and the generation of a subharmonic response. Hence, our hypothesis that hot spots are related to acoustic buckling could not be verified