Self-Assembled Polystyrene Nanospheres for Plasmonic Enhancement in Biosensor Applications

K. Smith, C. Norville, M. Windham, J.M. Dawson
West Virginia University,
United States

Keywords: plasmonics, fabrication, nanospheres, biosensors, nano-fabrication, fluoresence enhancement

Summary:

The field of portable, point-of care biosensing has expanded in recent years to encompass a wide range of application areas, with detection of targets accomplished by a variety of techniques. Application-specific needs of further reduced of limit of detection, per-unit cost, and operational complexity continue to drive research and development efforts in the biosensor market. Even though label-free detection methods have made inroads to the market space, label-based optical detection still remains popular due to many similarities and compatibilities with bench-top analysis technologies. Label-based visible spectrum bio-sensing systems allow for an instant intensity/wavelength based visual feedback method that is common with methods of spectroscopy and electrophoresis. The drawback of these labeled visible spectrum based bio-sensors is the limit of detection is ultimately determined by the noise floor of the optoelectronic detector component. This limitation can be overcome through the addition of periodic plasmonic nano-structures (PPNS) to the surface of the sensor system being monitored by the detector. Operationally, these PPNS increase the radiative decay rate of photons and confine photon emission to specific directions, increasing photons per unit time and per unit area reaching the detector. For operation in the visible wavelength range, these features are typically on the order of the label emission, wavelength and require repeatable nano-patterning techniques that can achieve critical dimensions within 3-5% of as-drawn geometries. In order to fabricate such dimensions, electron beam lithography (EBL) is commonly used due to its high-accuracy feature production. While highly accurate, such systems have low throughput capabilities that aren’t cost-effective for mass production. A faster, more cost effective method of fabrication utilizes polystyrene nano-spheres (NS) as the feature basis. These NS can be deposited easily using dip-coating techniques, and a controlled reduction in size to desired dimensions can be achieved using oxygen plasma processes. In this work, the development and fabrication of a PPNS using NS for enhancement of quantum dot emission at565nm wavelength is highlighted. A simulation analysis will show base feature dimensions of 150 nm diameter (125nm diameter NS and 25nm gold layer) provide a 300-fold enhancement of electric field strength near the plasmonic surface. Based on these results, PPNS were fabricated using self-assembled 200 nm diameter NS etched to 125 nm diameter, and then coated with a 25 nm Au layer. A cadmium selenide zinc sulfide (CdSe/ZnS) quantum dot (QD) solution was then used for emission enhancement characterization, resulting in a ~4-fold increase in detected emission over unpatterned metal regions.