# Molecularly Imprinted Filtering Adsorbents for Odor Sensing
## 3 Results and Discussion
### Controlling the Adsorption Properties of the PVC-DOP Adsorbent with a MIF  
To control the specificity of the gas adsorption, a MIFA composed of a PVC-DOP adsorbent with a MIF\({}_{\text{PAA}}\) layer was fabricated and its adsorption properties were evaluated. For this experiment, a PVC-DOP composite adsorbent containing 67% DOP was used. Two types of MIFAs were fabricated using different template materials (2-decanone and 2-nonanone), and an adsorbent without a MIF layer was used as a negative control. The amount of gas adsorbed by each adsorbent was measured using the GC-MS/SPME method. The sample gas consisted of a mixture of 2-decanone, 2-nonanone, hexanoic acid, nonanol, and 5-nonanone. For both MIFAs, the adsorption of the gas used as the template was increased compared with the amount adsorbed by the negative control (data not shown). In contrast, the amount of adsorbed non-template gases decreased, which shows that the MIF\({}_{\text{PAA}}\) layers behave as MIFs. These results show that the specificity of the gas adsorption was enhanced by addition of MIFs to the adsorbents. We also tested the gas specificity of the MIFA in detail using nine different gases (heptanal, hexanoic acid ethyl, anisole, propanoic acid, benzaldehyde, nonanol, methyl salicate, hexanoic acid, and guaiacol). The MIFA fabricated with a hexanoic acid template was used in this experiment. Figure 7 shows the normalized amounts of adsorbed gas relative to those for the adsorbent without a MIF. The amount of gas adsorbed increased only for hexanoic acid, and decreased for all of the other gases. The normalized amount of adsorbed target gas, hexanoic acid, was seven times greater than the adsorbed amounts of the other gases. The adsorption properties of PDMS-DVB adsorbents with a MIF\({}_{\text{MAA}}\) filtering layer were also evaluated. The sample gases, 2-heptanone, 1-heptanol, and heptanoic acid, were adsorbed onto the MIFA, which was prepared using heptanoic acid as the template molecule. The adsorption properties \begin{table}
\begin{tabular}{c c c} \hline  & **1-Heptanol/2-Heptanone** & **Heptanoic Acid/2-Heptanone** \\ \hline PDMS & 0.479 & 0.00215 \\ PDMS-DVB & 0.541 & 0.0229 \\ PEG-PVA & 1.779 & 0.046 \\ PDMS-PVA & 0.515 & 0.00647 \\ \hline \end{tabular}
\end{table} 
Table 2: Adsorption specificity of composite adsorbents. Adsorption amounts determined from the total ion current (TIC values) were normalized to the TIC values for adsorbed 2-heptanone on each adsorbent. Figure 7: Amounts of gas adsorbed by a molecularly imprinted filtering adsorbent fabricated using hexanoic acid as the template. The amounts are normalized relative to the amounts of gas adsorbed by a polyvinyl chloride–dicotyl phthalate (PVC-DOP) adsorbent without a molecularly imprinted filter. were measured using the same method as used for the other experiments in this paper. A non-imprinted filtering adsorbent (NIFA), which is an adsorbent made following the same procedure as that used to prepare MIFAs but without a template molecule, was prepared as a negative control. The normalized amounts of adsorbed gas, relative to the amounts adsorbed with the NIFA, are shown in Figure 8. The normalized amount of adsorbed gas increased for heptanoic acid but decreased for 2-heptanone and 2-heptanol. The ratios between heptanoic acid and 2-heptanone, and heptanoic acid and 2-heptanol, were 9.5 and 18, respectively. These results show that the MIF layer could filter out a wide range of gas species. Thus, considerable control over the gas adsorption properties of adsorbents could be achieved. In our previous study, we confirmed the regeneration and degradation of MIF layers [17]. The sensing system using MIFAs has a heating unit, and then the adsorbed gas can be desorbed in a higher temperature condition (80 \({}^{\circ}\)C). Such desorption was confirmed before GC-MS/SPME evaluation, i.e., template gas molecules cannot be detected after the 80 \({}^{\circ}\)C vacuum-heating process.