OVERVIEW

A novel sensor, the Surface Plasmon Band Gap Sensor (SPBG) for robust and versatile molecular sensing in field applications

We are developing a new sensor, the Surface Plasmon Band Gap Sensor (SPBG). It will permit to bring the specific qualities of Surface Plasmon Sensing, versatility and real-time sensing, to field applications like such encountered in deployed sensors networked. This novel sensor (patent pending: UCLA Case 2005-278-1) is based on the properties of the propagation of surface plasmons waves through nanostructures. This sensor can be used for monitoring chemical as well as biological species from molecules to living cells. We propose to specifically apply the capacities of this sensor for nitrate sensing and algae monitoring.

Overview of the problem

Importance of surface plasmon sensing for CENS

Surface plasmons are electromagnetic waves which propagate at optical frequencies on the interface between a metal and a dielectric. Their wavelength is extremely sensitive to any chemical change at the interface which will dramatically change their properties (frequency- wavelength relation). This is the working principle behind Surface Plasmon Resonance systems, a well-known and commercially available technique (aka Biacore® System) mainly used to measure protein/protein or RNA/DNA affinities. Unlike most biological detectors and some chemical sensors, Surface Plasmon sensors operate in real-time and are versatile and label-free. Being label-free means that they don’t necessitate a so-called sandwich-assay, a time consuming technique where the target molecule has to be sandwiched between a capture molecule and a signaling molecule to be detected. The key motivation for developing such an optical censor for field application like those developed in the CENS is versatility. Once a censor has been developed for targeting one component, it is straight-forward to change the functionalization of the sensing interface to target a different molecule. Furthermore, surface plasmon censors don’t require complex optical systems and can be mass-produced at a low price.

Specific Problem statement

Despite their apparent simplicity, commercially available SPR systems are bulky and extremely fragile and are only intended for laboratory-use. More specifically, what is measured in such a system is the coupling intensity between photons and surface plasmons, which can only be achieved when transverse momentums are matched. Controlling the transverse momentum in a robust manner either in bulk or in a waveguide is very difficult, making improbable the field deployment of SPR sensors.

Proposed APPROACH

To overcome these obstacles, we are developing an entirely new concept of sensor, the SPBG. It is not a miniaturized version of an established technique. It is an entirely novel technique (patent pending: Case 2005-278-1) based on the measure of the propagation of surface plasmon waves through periodic nanostructures(Yoon 2003). It does not necessitate a precise control of the transverse momentum of the photons and it is therefore miniaturizable. It will share most of the attractive features of SPRs, providing a robust, versatile and real-time sensing that can be deployed on the field.

SYSTEMS / EXPERIMENTS

Our sensor development is articulated around four major components. See Table 2 for the completion status

Numerical simulations

The purpose of the numerical part was to demonstrate the concept of the sensor and to optimize the characteristics of the experimental setup in order to decrease the development time. We implemented two codes: one simulated the behavior of electromagnetic fields on arbitrary interface (C method)(Li 1999) and the other simulated a stack of periodically modified layer (RCWA)(Moharam 1995). The numerical simulations have been extremely promising and they proved not only the concept of our sensor but also enabled the comparison of the performances of our sensor versus existing sensors. Figure 2 is an example of the numerical results obtained.

Surface plasmon measurement system

We implemented some critical improvements to the home-made optical goniometer that we constructed during the first year. The most drastic change is that it is now possible to directly measure the reflection of a beam at several angles of incidence and map it on a screen. Also, thanks to a redesigned fluidic cell, it is now possible to probe the sample at higher angles and at several locations on the sample. Next is a picture of the system setup. The modified goniometer allows the measure in real time of the surface plasmon resonance angle without scanning. Because of the vertical translation invariance, it also enabled the possibility of having a multi-channel sensor to detect different chemical markers as the same time.

Nanostructures fabrication

The nanostructure is a critical part of the system and its fabrication yielded to many challenges. Because of their subwavelength size, it is not possible to use traditional lithography technique for the fabrication. We tried out three different techniques to achieve the fabrication of the nanostructures vital to our sensor.

E-beam lithography (EBL)

This is the traditional way to fabricate nanostructures in the IC industry. Although it is very reliable, it is quite expensive and large surfaces cannot be achieved at low cost

Nanoimprinting.

This technique is similar to what is known as embossing in the macro world. It is essentially a reproductive technique which has a proven track record. However, our numerical simulations show that the type of nanostructures this technique can produce does not optimize the performance of our sensor .

Nanostamping

This is by far the cruder technique (Xia 1998). A stamp is made out of a polymer and is used to fabricate the nanostructures by chemical transfer. This technique will most likely be used in our censor fabrication.

Surface Functionalization

It is necessary to functionalize the surface to permit specific detection. We developed a new technique, LB-SAM to be used to coat metallic surfaces to make them active for surface plasmon detection of nitrate. The results have been already presented in the precedent annual reports. For algae detection, we are going to use specific antibodies. Coating metallic surfaces with antibodies is a routine procedure and should not present too many problems.

PEOPLE

Faculty:

Prof. Chih-Ming Ho

Graduate Student:

Arnaud Benahmed