# Detection of Bacteria on Metal Microarrays

Shaun Williams, PhD

### Research Goals

1. Determine how far the detection limit could be pushed
• how small of a bacterial number density can be detected.
2. Record absorption spectra for a variety of bacteria
• Begin building a bacteria spectral library

### First Mesh of the Summer

Reproducibility Test

### A Big Problem

• Same original mesh dimensions
• Same deposition time
• Time was measures with wristwatch
• Accurate to at best a half-second
• Better control of deposition time is clearly needed

### A Big Problem

• Same original mesh dimensions
• Same deposition time
• Time was measures with wristwatch
• Accurate to at best a half-second
• Better control of deposition time is clearly needed

### A Big Problem

• Same original mesh dimensions
• Same deposition time
• Time was measures with wristwatch
• Accurate to at best a half-second
• Better control of deposition time is clearly needed

### Design of a Timer

• Arduino electronic control board (bottom in image)
• Hand coded
• Time accuracy of 1 millisecond
• Control breadboard (middle in image)
• Power controller (top in image)
• Program triggers this on an off as appropriate

### Comparison of Mesh with New Timer

• Both deposited for 6 seconds (6000 ms)
• How can this difference be explained?

### How Big is this Difference

• This difference isn't as big as it seems compared to the original nickel mesh
• The overall dominant peaks at $$\sim 750\,\mathrm{cm}^{-1}$$ has a %T of ~70% in the original nickel mesh.
• This amounts to percent changes of 99.1% and 99.9% respectively in %T.
• The dominate peak at $$\sim 1450\,\mathrm{cm}^{-1}$$ has a %T of ~7% in the original nickel mesh.
• This amounts to percent changes of 84% and 90% in %T.

### Explanation of the Differences

• Let's compare the surface area of the mesh in these two case.
• Green spectra: $$0.0195\,\mathrm{in}^2 = 0.126\,\mathrm{cm}^2$$
• Purple spectra: $$0.0272\,\mathrm{in}^2=0.175\,\mathrm{cm}^2$$

• The distance between the anode (the mesh) and the cathode is approximately $$0.102\,\mathrm{in}=2.59\,\mathrm{mm}$$ during deposition and the potential difference between the two is $$10.0\,\mathrm{V}$$.
• This yields an electric field strength between the anode and cathode of $$3860\,\bfrac{\mathrm{V}}{\mathrm{m}}$$ or $$3.86\,\bfrac{\mathrm{kV}}{\mathrm{m}}$$.
• Hypothesis: Larger surface area means that the maximum electric field strength is in a larger volume of the solution. Therefore, more deposition.

### Spectrum of Micrococcus luteus

• A program was written that allows for the automated removal of the background spectrum
• The program eliminated the need to extract the background point-by-point, spectrum-by-spectrum.

### Future Work

1. Further improve the transmission-to-absorption program to further eliminate artifacts.
2. Push the detection limits to see how small an amount of bacteria can be detected.
3. Analyze as many bacteria as possible.

/