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

Multiplexed identification, quantification and genotyping of infectious agents using a semiconductor biochip

The emergence of pathogens resistant to existing antimicrobial drugs is a growing worldwide health crisis that threatens a return to the pre-antibiotic era. To decrease the overuse of antibiotics, molecular diagnostics systems are needed that can rapidly identify pathogens in a clinical sample and determine the presence of mutations that confer drug resistance at the point of care. We developed a fully integrated, miniaturized semiconductor biochip and closed-tube detection chemistry that performs multiplex nucleic acid amplification and sequence analysis. The approach had a high dynamic range of quantification of microbial load and was able to perform comprehensive mutation analysis on up to 1,000 sequences or strands simultaneously in <2 h. We detected and quantified multiple DNA and RNA respiratory viruses in clinical samples with complete concordance to a commercially available test. We also identified 54 drug-resistance-associated mutations that were present in six genes of Mycobacterium tuberculosis, all of which were confirmed by next-generation sequencing

Multiplexed identification, quantification and genotyping of infectious agents using a semiconductor biochip

The emergence of pathogens resistant to existing antimicrobial drugs is a growing worldwide health crisis that threatens a return to the pre-antibiotic era. To decrease the overuse of antibiotics, molecular diagnostics systems are needed that can rapidly identify pathogens in a clinical sample and determine the presence of mutations that confer drug resistance at the point of care. We developed a fully integrated, miniaturized semiconductor biochip and closed-tube detection chemistry that performs multiplex nucleic acid amplification and sequence analysis. The approach had a high dynamic range of quantification of microbial load and was able to perform comprehensive mutation analysis on up to 1,000 sequences or strands simultaneously in <2 h. We detected and quantified multiple DNA and RNA respiratory viruses in clinical samples with complete concordance to a commercially available test. We also identified 54 drug-resistance-associated mutations that were present in six genes of Mycobacterium tuberculosis, all of which were confirmed by next-generation sequencing

On the impedance of the silicon dioxide/electrolyte interface

The small-signal impedance of electrolyte/insulator/silicon structures is partly determined by the properties of the insulator/electrolyte interface. A theoretical model for this interfacial impedance is derived. Two parallel contributions are involved: the double-layer capacitance, for which a Gouy-Chapman-Stern model is adopted, and a branch containing the capacitance related to the surface reactions with H+ and OH− ions from the electrolyte. These surface reactions cause the total interfacial impedance to be very low for insulators with a high surface reactivity such as, for instance, Al2O3 or Ta2O5. For SiO2 surfaces, the reactivity is much lower, implying a larger interfacial impedance. Measurements of the interfacial impedance were carried out at low frequencies on 12 nm SiO2 layers in NaCl electrolytes at ionic strengths of 10−4, 10−3and 10−2 M. The results agreed with the theoretical predictions which were based on parameter values obtained from independent measurements of ψ0/pH characteristics. The agreement confirms the model for the formation of the surface charge through reactions of fixed silanol groups in the SiO2 surface

The role of buried OH sites in the response mechanism of inorganic-gate pH-sensitive ISFETs

The models proposed in the literature on the mechanism of operation of inorganic-gate pH-sensitive ISFETs can be divided in three categories: those involving changes at the Si/insulator interface, those involving bulk ionic diffusion and those based on reactions of surface sites. The first two categories imply a time response limited by diffusion through the gate insulator. Time response data on Al2O3-gate ISFETs show that the inrtinsic response time is of the order of a few milliseconds or faster. Published data for other insulators are similar. The diffusion coefficient for H+ diffusion in SiO2 is much too low to explain this fast response, and for Al2O3 and Si3N4 no H+ movement can be detected at low temperatures. Gel layer formation cannot increase ionic mobility sufficiently to explain the observed response times. Therefore we conclude that surface effects must be responsible for the fast pH response. We propose that an additional slow response resulting in hysteresis as observed in SiO2-gate ISFETs, as well as a decreased sensitivity for higher pH values, are due to the presence of OH sites buried beneath the surface. These interior OH sites can be created by steam oxidation or by exposure to the aqueous electrolyte