Speaker – Ankit Arora (EE13S002)
Is it possible
- to fabricate low-cost, use & throw SERS substrates
- over large areas (few square cms)
- without using any nanofabrication equipment or high vacuum processes
- without using commercial nanolithography
- without need for fabrication expertise
- low volume (< Nano litre), low concentration (10 nano Molar) analyte sensing
- Raman enhancement of ≥ 104
- few molecule detection
- potential for commercialization
The brief answer is YES. We believe that we have developed a very simple method to achieve this.
Surface Enhanced Raman Spectroscopy (SERS) is a powerful vibrational spectroscopic technique that makes use of enhanced electric field confinement in the vicinity of analyte molecules of interest. The enhanced electric field confinement may be achieved using metal nanostructures having resonant or non-resonant hot-spots. Commercial SERS substrates consist of roughened Silicon coated with a noble metal, utilizing resonant hot-spots. In academic research, modern nanofabrication techniques such as e-beam lithography and focused ion-beam patterning are used to fabricate SERS substrates. Neither of these approaches are suitable for a low cost, large area, use & throw, resource constrained setting without the use of nanofabrication equipment.
In this work, two simple unconventional fabrication techniques have been explored for the fabrication of low cost, large area, use & throw SERS substrates:
i. Gold coated Self-assembled colloidal crystal and Nanosphere Lithography (Non-resonant hot-spot based) [were found to be not so great]
ii. Resonant hot-spot films with wide tunability on glass substrates
Using the first method, and a Langmuir transfer method followed by a lift-off and metallization, substrates with Raman intensity enhancement of > 1.7 × 104 were obtained. Though the method was low-cost and showed high enhancement, it involved significant polymer and surface chemistry, and process control was very difficult due to large variances due to change in minute experimental conditions.
Therefore, in search of extremely simple method involving no chemistry, we investigated the possibility of fabrication of large area, resonant hot-spots on glass substrates using simple physical techniques. Such films showed minimum enhancement of > 3 × 104 with 10−5M Rhodamine 6G as Raman probe and were able to detect low concentration (≈ 10−8M) low volume (≈ 200 pico litre) analyte.
Finally, these substrates were used for testing thioglycolic acid monolayer functionalization of the substrates for potential in bio-sensing applications.
We show that films fabricated using our method are the easiest to fabricate over large areas as of today and they are probably the most affordable SERS substrates with comparable enhancements to a commercial substrate.