JEZZ LECKENBY
The need for fast and reliable biosensor arrays for disease screening is growing. A method of biological species detection developed by the University of Strathclyde’s (Glasgow, UK) Department of Pure & Applied Chemistry shows promise for answering that need. The approach uses nanostructured plasmonic gold surfaces created by surface-enhanced resonance Raman scattering (SERRS). Careful, directed placement of a biological species or capture chemistry in an array facilitates efficient reading via ultrafast Raman line mapping. The process also enables transition from serial placement of biological species/capture chemistry to a massively parallel deposition method, and this flexibility enhances the throughput by many orders of magnitude.
Precision placement in the array is enabled by Dip Pen Nanolithography (DPN) a process patented by NanoInk (Skokie, IL) for direct-write deposition of nanoscale materials onto a substrate. The process can use pyramidal scanning-probe microscope tips, hollow tips, or tips on thermally actuated cantilevers to deposit a wide variety of chemical inks onto numerous substrates. In fact, DPN can deposit multiple chemical compounds on the same substrate with nanoscale registration.
The approach was designed to provide simple operation and straightforward experimental procedures. DPN works without resists in ambient conditions (no UHV requirement). NanoInk’s DPN tool, NSCRIPTOR, can perform both patterning and imaging in a scalable manner. Commercially available linear 1-D parallel pen arrays can extend up to several hundred cantilevers, while NanoInk’s 2-D nano PrintArray tool equips NSCRIPTOR to provide massively parallel DPN patterning with two-dimensional arrays of 55,000 pens.
Producing the arrays
In seeking to develop novel biosensor arrays, Duncan Graham, Strathclyde professor of chemistry, learned of his colleague Rob Stokes’ enthusiasm for DPN. Stokes’ previous research work had used SERS in conjunction with optical microscopy and SEM using a substrate called Klarite. Produced by etching silica, it comprises pyramidal pits with 1.3 µm base and a depth of 1 µm which are then coated in gold. Now commercially available, (D3 Technologies, Glasgow, UK), Klarite substrates are used for surface-enhanced Raman spectroscopy (SERS) offering superior reproducibility and signal consistency and enabling rapid detection to parts per billion. D3 has unique surface chemistry to maximize the potential of SERRS as a detection technique and is developing clinical diagnostic products with superior performance.
Strathclyde’s early DPN work used standard chemistry protocols developed by NanoInk’s team of chemists, for example writing with DNA. Now, the group develops its own methodology. Stokes sees his work using Klarite a bit like working with a surface plasmon resonance (SPR) chip. He coats it through controlled self-assembled monolayer chemistry, using cyclic dithiols that offer strong surface affinity and have no spacer groups.
Using DPN, it is possible to deposit directly into these pits and make subsequent spectroscopic measurements using SERS. DPN enables close control of this process, varying parameters such as shape, size and quantity of molecules, with nanoscale precision. This is enabling the creation of micro and nano array structures to study different types of molecular interactions, for instance DNA-protein, and protein-protein. Using SERS, it is possible to follow reactions; to define which materials interact with what.
The ability to draw lines enables fast throughput with spectrometers that read 42 lines at once. Rob appreciates the NSCRIPTOR’S three-point leveling and speed control functions, which help avoid scratching coatings in the wells. He says scalability and sensitivity enable information density.
Looking to the future, Graham and Stokes see more synergy between applied chemistry and spectroscopy. At present, the resolution limit is approximately 10nm; they aim to eventually enable single particle studies. In particular, they see potential for applied DPN as a manufacturing tool: to apply printing functionality to produce biodetector systems.
Jezz Leckenby is director and cofounder of NetDyaLog Ltd.