A Microstrip based RF Filter for Biosensor Applications

Abstract

There is need in medical diagnostics for accurate, fast, and inexpensive devices, which can be routinely used. In this context, micro-biosensors are considered to provide viable solutions to the problems posed by the current healthcare industry. This is because these biosensing devices offer considerable advantages, such as specificity, small size, faster response, and low cost. Hence, innovative technique is desirable such as microstrip technology, which is a good means of employing planar and miniaturized high frequency filter designs. The advantages of implementing a high frequency filter design using microstrip technology includes low cost, light weight, compact size, planar structure and easy fabrication and integration with other components when deployed as a biosensor. Designing a highly sensitive and selective sensing element of a Biosensor is the aim of this research. To achieve this task a 5 th and 7th order Chebyshev type low pass filter possessing a passband ripple of 1dB and a 3rd and 5th order Chebyshev type Bandpassfilter possessing a bandwidth of 0.5GHz, a fractional Bandwidth of 20% and a centre frequency of 2.5GHz were designed. A second fabrication run was used to fine tune the device design and test point on the device. Three sets of microstrip filters were produced, two of these were on a quartz substrate using two distinct materials, one of these materials is the chemically reduced graphene oxide (rGO), produced from the hydrazine reduction of graphene oxide, while the second filter produced on a quartz substrate is the one made from a nano gold film material this was being produced by gold deposition technique on the quartz substrate, the third of the three set is the microstrip filter produced on an FR4, this was made from a laser ablation technique resulting in a laser inscribed graphene (LIG). For the first two cases, mask of the designed geometry was used to precisely implement the filter design on the substrate, while for the LIG microstrip filter, the design was engraved on a Kapton tape using a laser machine. The conductivity of the rGO was observed to have a maximum value of 8.7mS/m, while that of the gold film material is known to be 45.2 x106 S/m, and the conductivity of the LIG was observed to be 0.28mS/mm. The sensor’s RF characteristics was investigated using a vector network analyser (VNA), while ANSYS and Sonnet Lite simulation tools indicate the potential for rGO material, but very good results were recorded for the gold film material, while the LIG results indicated the need for improved conductivity. The gold 5 th order bandpass filter (5BPF) filter showed best repeatability with a frequency of 2.38GHz and standard deviation in the resonant frequency measurements of a single device of +/- 0.19MHz. Its initial functionalisation and then monolayer coverage of the sensor with a layer mouse IgG indicated that the corresponding shift in frequency response occasioned by the presence and volume of the target sample is an indication of the system’s selectivity and suitability for deployment for biological sensing application. Plans are currently on the way to test more biological samples with lower concentration levels to verify the filter’s sensitivity, selectivity, and wide range applicability as a biosensor sensing element. The future areas to be addressed are to enhance the fabricated material’s property and sensor device miniaturisation.