An electrochemical comparison of thiolated self‐assembled monolayer (SAM) formation and stability in solution on macro‐ and nanoelectrodes

Thiolated self-assembled monolayers (SAMs) formed on metal electrodes have been a topic of interest for many decades. One of the most common applications is in the field of biosensors, where this is a growing need for functionalising nanoelectrodes to realise more sensitive and implantable sensors. For all these applications the SAM functionalised nanoelectrodes will need to make reliable and interpretable electrochemical measurements. In this work, Electrochemical Impedance Spectroscopy (EIS) is used to monitor both the formation and subsequent stability of 6-mercaptohexan-1-ol SAMs on macro and nanoelectrodes and compares the two. To develop effective devices, it is crucial to understand both SAM formation and the resulting signal stability on nanoscale surfaces and this is done by comparing to behaviours observed at the well understood macroscale. We report an initial stochastic binding event and subsequent re-arrangement of the SAMs for both electrode types. However, this re-arrangement takes hours on the macro scale electrodes but only seconds on the nanoelectrodes. This is proposed to be due to the different structure of the SAMs on the electrodes predominantly driven by their bulk to edge ratios. After formation, the SAMs formed on both macro and nanoelectrodes exhibit significant instability over time. The reported results have practical implications for the construction of SAM based biosensors on macro and nanoscale electrodes

Functionalised Microscale Nanoband Edge Electrode (MNEE) Arrays; the systematic quantitative study of hydrogels grown on nanoelectrode biosensor arrays for enhanced sensing in biological media

We demonstrate a hydrogel-coated nanoelectrode array biosensor that is resistant to biofouling.</p

Impedimetric measurement of DNA–DNA hybridisation using microelectrodes with different radii for detection of methicillin resistant Staphylococcus aureus (MRSA)

Due to their electroanalytical advantages, microelectrodes are a very attractive technology for sensing and monitoring applications. One highly important application is measurement of DNA hybridisation to detect a wide range of clinically important phenomena, including single nucleotide polymorphisms (SNPs), mutations and drug resistance genes. The use of electrochemical impedance spectroscopy (EIS) for measurement of DNA hybridisation is well established for large electrodes but as yet remains relatively unexplored for microelectrodes due to difficulties associated with electrode functionalisation and impedimetric response interpretation. To shed light on this, microelectrodes were initially fabricated using photolithography and characterised electrochemically to ensure their responses matched established theory. Electrodes with different radii (50, 25, 15 and 5 µm) were then functionalised with a mixed film of 6-mercapto-1-hexanol and a thiolated single stranded ssDNA capture probe for a specific gene from the antibiotic resistant bacterium MRSA. The complementary oligonucleotide target from the mecA MRSA gene was hybridised with the surface tethered ssDNA probe. The EIS response was evaluated as a function of electrode radius and it was found that charge-transfer (RCT) was more significantly affected by hybridisation of the mecA gene than the non-linear resistance (RNL) which is associated with the steady state current. The discrimination of mecA hybridisation improved as electrode radius reduced with the RCT component of the response becoming increasingly dominant for smaller radii. It was possible to utilise these findings to produce a real time measurement of oligonucleotide binding where changes in RCT were evident one minute after nanomolar target addition. These data provide a systematic account of the effect of microelectrode radius on the measurement of hybridisation, providing insight into critical aspects of sensor design and implementation for the measurement of clinically important DNA sequences. The findings open up the possibility of developing rapid, sensitive DNA based measurements using microelectrodes

Integration of Electrodeposited Ni-Fe in MEMS with Low-Temperature Deposition and Etch Processes

This article presents a set of low-temperature deposition and etching processes for the integration of electrochemically deposited Ni-Fe alloys in complex magnetic microelectromechanical systems, as Ni-Fe is known to suffer from detrimental stress development when subjected to excessive thermal loads. A selective etch process is reported which enables the copper seed layer used for electrodeposition to be removed while preserving the integrity of Ni-Fe. In addition, a low temperature deposition and surface micromachining process is presented in which silicon dioxide and silicon nitride are used, respectively, as sacrificial material and structural dielectric. The sacrificial layer can be patterned and removed by wet buffered oxide etch or vapour HF etching. The reported methods limit the thermal budget and minimise the stress development in Ni-Fe. This combination of techniques represents an advance towards the reliable integration of Ni-Fe components in complex surface micromachined magnetic MEMS

Test structure and measurement system for characterising the electrochemical performance of nanoelectrode structures

This paper presents a complete test structure and characterisation system for the evaluation of nanoelectrode technology. It integrates microfabricated nanoelectrodes for electrochemical measurements, 3D printing and surface tensionconfined microfluidics. This system exploits the inherent analytical advantages of nanoelectrodes that enables their operation with small volume samples, which has potential applications for onwafer measurements