pH sensing through a single optical fibre using SERS and CMOS SPAD line arrays
K. Ehrlich, A. Kufscák, S. McAughtrie, H. Fleming, N. Krstajic, C. J. Campbell, R. K. Henderson, K. Dhaliwal, R. R. Thomson, and M. G. Tanner
Heriot-Watt University / University of Edinburgh
The paper explores the ability of our advanced in-house made detectors to collect the arrival time of single photons and discusses how this additional information can be used for sensing pH in remote and size restricted areas such as the distal lung
pH is a measure of how acidic something is, and is therefore a very important parameter for the human body. Inflammation and cancerous environments can change healthy pH values in the body. Knowing the pH of a suspicious region will help clinicians in their diagnosis and choice of treatment. However, most regions inside the human body are very difficult to reach because they are remote and small in size. Optical fibres, which can be as thin as human hair and extremely flexible, are able to reach these areas; for example, in the alveoli space in the lung. We deposit tiny spheres on the tip of such a fibre and use them as pH sensors.
Our aim was to miniaturise and simplify the probe design and to improve the accuracy of pH sensing by using a time-resolved methodology: For every photon that is generated, we are measuring not only the wavelength but also the point in time when it was generated. This additional information can be used to divide the signal into wanted and unwanted parts. The wanted part contains the information from the pH sensitive molecule. Applying this technique, we are able to present a miniaturised sensing probe.
Exciting a molecule with light instantaneously leads to a scattering process, in which the molecule re-emits light at a very specific wavelength. This process, called Raman scattering, can expose the ‘fingerprint’ of any molecule. As Raman signals are extremely weak, we are combining two methods in this paper to enhance the Raman signal. First, the molecule under investigation is attached to gold nanoshells which are deposited on the tip of the optical fibre. The gold substrate enhances the signal through an effect which is called surface-enhanced Raman spectroscopy (SERS). The emitted light from the molecule is sensitive to its environment and changes with varying pH level. Secondly, we are using a self-built spectrometer system to record the light emitted from the molecule and all unwanted background light which can never be completely avoided, in a time-resolved manner. This is done by using a special sensor which has the capability to count photons on all 256 pixels at the same time, as well as recording the arrival time of each individual photon. This means that we are able to identify the point in time when the light is emitted and filter out the unwanted background light from the desired Raman signal by using the different time scales these signals originate in.
As an example, the system was built for sensing pH, a key physiological parameter. Resolving the whole measurement on a time axis enables us to differentiate between the unwanted background and wanted Raman signal and enhances the visibility of the Raman signal significantly. The design of the pH sensor is simple and miniaturised to allow access to size-restricted regions inside the human body.