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Figure 2. Experimental setup used to measure the optical reflectivity spectra of porous silicon microcavity.
The functionalized PSi samples work as active substrates for the TMBP protein: on each
PSi optical structures were spotted 20 μl of a 173 μM TMBP solution in 2mM phosphate
buffer solution (pH 7.3) and incubated the system at 4 °C over night. After incubation and
three washing steps of five minutes each, used to remove the excess of biological matter
non covalently linked to the PSi surface, the presence of the protein in the spongy
structure was optically verified as a red shift of about 60 nm in the reflectivity spectrum in
the case of PSMC.
Experimental results and discussion
When recorded at the temperature of 25 °C, the PSMC characteristic resonance peak
undergoes only very small red shifts, of the order of 1 nm, on exposure to glucose
solutions with increasing concentrations, up to 200 μM, according to the low affinity of
TMBP with glucose at room temperature [7]: at this temperature few proteins can rearrange
their conformational structure and efficiently bind the ligand. The results of this
biomolecular interaction change dramatically if the same measurements are performed at
60 °C: in Figure 3 is reported the peak shift of the functionalized microcavity on exposure
to a 200 μM glucose concentration. The optical resonance undergoes a 4.5 nm shift
toward lower wavelengths.
Reflectivity (a.u.)
1.0
0.8
0.6
0.4
0.2
Unperturbed
After exposure to a 200 μM glucose solution
960 980 1000 1020 1040 1060 1080
Δλ
Wavelength (nm)
Figure 3. PSMC resonance blue shift on exposure to a glucose solution of 200 μM concentration.
The dose-response curve is shown in Figure 4: the curve is linear in the range of glucose
concentrations between 0 and 200 μM, with an estimated sensitivity to solute
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