# Cyclodextrin Polymers as a Promising Drug Carriers for Stabilization of Meropenem Solutions
## 3 Results and Discussion
### The Structure of HPCD-Polymers  
In order to prove the chemical structure of the obtained polymers, we have used ATR-FTIR and \({}^{1}\)H NMR spectroscopy. In the FTIR spectra of HPCD, we observed the most intense absorption bands in the 1100-1000 cm\({}^{-1}\) region corresponding to C-O-C glycoside bonds (Figure 2A) [23]. The formation of HPCD-CA leads to the appearance of bands 1725 cm\({}^{-1}\) and 1226 cm\({}^{-1}\) regarding to C=O in carboxylic and ester groups and C-O-C in ester groups, respectively [24]. Interestingly, the CA spectrum contains the band at 1720 cm\({}^{-1}\) (C=O st). The shift of the band to the higher wave numbers in HPCD-CA spectrum indicates the formation of ester bonds that was demonstrated, for instance, via chemical linkage between starch and CA in [25]. Figure 2: Normalized FTIR spectra of **(A)** HPCD, HPCD-Succinic acid polymer, HPCD-Critic acid polymer, and citric acid; **(B)** HPCD-HMD, HPCD-HMD-MP\({}_{\text{im}}\), and MP in the range of 2200–900 cm\({}^{-1}\), H\({}_{2}\)O, 22 \({}^{\circ}\)C.  
For HPCD-SA, we observed a similar pattern as for HPCD-CA (Figure 2A). The appearance of an intense band at 1717 cm\({}^{-1}\) corresponding to the C=O group in the polymer network. Thus, the formation of two ester polymers was confirmed. In the case of the HMD cross-linker, we expected the formation of a polyurethane bond with a typical band around 1740-1700 cm\({}^{-1}\) of the C=O group in the FTIR spectra [26; 27]. Indeed, for HPCD-HMD, we observed a wide band in the region of 1740-1690 cm\({}^{-1}\) (Figure 2B). Moreover, the 951 cm\({}^{-1}\) band appears, which corresponds to the C-N bond in primary amines [28]. This functional group might appear via the hydrolysis of unreacted isocyanate groups in aqueous media that was demonstrated earlier for cross-linked hydroxymethyl starch nanocapsules [27]. In addition, the positive \(\zeta\)-potential of the HPCD-HMD confirms the presence of -NH\({}_{2}\) groups on the surface of the particles (Table 1). In the FTIR spectra of HPCD-HMD-MP\({}_{\text{im}}\) (Figure 2B), we observed the bands that are characteristic for both MP and HPCD-HMD polymers. MP probably does not influence the synthesis process of polymeric networks. We suppose MP is physically entrapped into the polymer matrix on molecular level and is not chemically modified. Thus, MP retains its antibacterial activity. For a deeper study on how the imprinting affects MP's state in HPCD-HMD-MP\({}_{\text{im}}\) (MP\({}_{\text{im}}\)), the FTIR spectrum of HPCD-HMD was subtracted from the FTIR spectrum of HPCD-HMD-MP\({}_{\text{im}}\) (Figure S2). We found that the MP\({}_{\text{im}}\)'s FTIR spectra is similar to the FTIR spectra of free MP. The determined MP's encapsulation efficiency is 82 \(\pm\) 4% (compared to the initial C\({}_{\text{MP}}\) used in the synthesis), which means that the major part of drug molecule was imprinted into the polymer matrix. As such, the carrier's capacity is 0.13 mg per 1 mg of the sample. Previously, similar inclusion efficacy was obtained for imprinted mozifloxacin; more than 80% is immersed in the polymer structure [22]. Surprisingly, we observed two pronounced changes in the FTIR spectra of MP\({}_{\text{im}}\); the structural changes in the peak in the 1700-1500 cm\({}^{-1}\) region and the decrease of the 1750 cm\({}^{-1}\) peak's intensity. We will describe it in more detail as it reveals the most interesting changes in the MP's microenvironment upon complex formation. The effect of 1750 cm\({}^{-1}\) band intensity decreasing might indicate the participation of MP's carboxylic group in hydrogen bonding via complex formation with HPCD polymer matrix, as it was demonstrated previously for MP-CD complex in [2]. If this band corresponds to the carboxylic group, this spectral region has to be pH-sensitive. In the independent experiment, we compared spectral patterns for MP solutions with various pH values (Figure 3). According to the spectral data, this region is definitely pH-sensitive.