The remaining Al was selectively dissolved to ensure that the reflection observed was only due to the rugate structure. Figure 4a shows the resulting reflectance spectra. The spectra displayed a well-defined band without sidelobes as we expected from the apodization of the current profile. We observed that the pore-widening treatment resulted in a blueshift of the reflection band as Protease Inhibitor Library concentration well as a lower reflection below and above the band. This is the result of the partial dissolution of the alumina, which decreases the overall refractive index of the rugate filter. A more interesting fact is how the band widened after the pore-widening treatment. This broadening is related to the refractive

index contrast of the rugate filter (Δn). The higher the Δn, the wider the band. This is in good agreement with our previous reported results for NAA obtained with periodic anodization voltages [7, 14]. Analysis of the transmittance measurements (Figure 4b) showed how the pore-widening post-treatment led to less steep edges

in the stop band, possibly due to scattering and absorption CHIR 99021 of the alumina. Figure 4 Reflectance and transmittance characterization of the NAA rugate filters. (a) Reflectance and (b) transmittance spectra of NAA rugate filters anodized for 300 cycles, with an apodized sinusoidal current profile with a period time of T = 200 s. Real-time sensing As a proof of the possible application of this structure, we performed a sensing experiment in a flow cell and monitored the position of the reflectance band in real-time for a sample fabricated Erlotinib molecular weight with a period time of T = 200 s, a total of 300 cycles, and a pore-widening post-treatment of t pw = 5 min (Figure 5). After acquiring a reference of the sample in air, we flowed EtOH at a rate of 1 mL min−1. Then, we flowed deionized water and, finally, EtOH again in order to prove the repeatability of the measurement. The results presented in Figure 5 show a highly stable signal with no significant drift within the time range and a very low noise of about 0.04 nm. The NAA rugate filter was able to distinguish

between two liquids with a similar refractive index (n water = 1.333, n EtOH = 1.362) with a sensitivity of 48.8 nm/refractive index unit (RIU). Moreover, when EtOH was reintroduced into the chamber, the position of the reflection band returned to the same value of the first EtOH infiltration, indicating the high reproducibility of the results. Figure 5 Sensing results. Real-time measurement of a NAA rugate filter in a flow-cell where EtOH, deionized water, and EtOH were serially flushed in to the chamber. Conclusions NAA rugate filters were fabricated using a current control method based on a sinusoidal current profile with a maximum amplitude of just 1.45 mA cm−2. Thanks to this small current peak-to-peak value, the voltage was contained within 40 ± 5 V.