As previously stated, starch nanoparticles (SNPs) are distinct from starch nanocrystals but are also referred to as starch nanocrystals (SNCs). The crystalline fraction of starch granules that remains following acid or enzymatic hydrolysis or other mechanical processes is referred to as SNCs. However, the word starch 'nanocrystals' refers to solely the crystalline section, while 'nanoparticles' can also encompass amorphous portions [28]. Starch can be modified to change its morphology by influencing hydrogen bonding in a regulated pattern. The mechanical modification includes altering the characteristics of starch in a regulated manner using heat, friction from shaking, and moisture. The change alters the features of starch granules such as roughness, amylose content, and swelling strength. In addition, it encompasses the solubility index and the amount of water absorption capability [12, 28, 29].

Modification can be accomplished chemically by inserting a functional group into the starch molecule. This aids in bringing out the many changes in the starch molecule's varied features. Granular modifications of native starches can occur in chemical modifications, such as acid modification of starch, by treating the starch below its gelatinization temperature in an aqueous acid mixture, hydrolysis reduces the size of the granule as well as paste viscosity. Native starch is a semicrystalline polymer with different degrees of crystallinity because it is structured in alternating amorphous and crystalline lamellae in the granules. The amylopectin component of starch is solely responsible for crystallinity, whereas amylose is primarily responsible for amorphous areas [14, 28]. Amylose quantity reduced somewhat after acid alteration, while starch kept its native crystallinity pattern. The acid mostly affected the starch granule's amorphous areas [12, 30]. When a native starch is exposed to acid hydrolysis, the starch granules normally dissolve at the amylose area, while the amylopectin sections remain unchanged [31]. In conclusion, the development of SNP and SNC results in a decrease in amylose concentration because hydrolysis of starch by acid or enzymes causes a random decrease in chain length [12].

Morphology and particle size of starch nanoparticles

The top-down procedure was applied in both the chemical and mechanical methods of preparing starch nanoparticles in this study, in which nanoparticles were formed from structural and size refinement through a breakdown of bigger particles. The morphology and particle size of the starch nanoparticles generated were determined using SEM. CTSN and MTSN had diameters smaller than 1000 nm when compared to native or untreated starch, demonstrating the formation of sweet potato starch nanoparticles.

Scanning electron micrographs (Fig. 1) revealed that the particle appearance of MTSN was unaffected. The shape of the MTSN starch nanoparticles changed from spherical to irregular. In the literature, starch nanoparticles are often round, uneven, and rodlike in shape [32]. The incubation of the native starch in moisture with constant shaking and heating at 40 °C resulted in the diminution of the particle size of the starch granules due to mechanical friction from continuous shaking breaking the covalent bonds of the starch particles. For processing times of 3 and 6 days, the size of native starch declined from the micro-range to the nano-range, resulting in starch nanoparticles (Table 1). The granular size of the MTSN processed in 3 days (683.35 nm ± 29.45) was smaller than the granular size of the MTSN processed in 6 days (729.03 nm ± 47.05).

Table 1 Properties of ciprofloxacin-loaded untreated and treated potato starch

SEM revealed that the untreated starch granules had some mild aggregation, but the treatment methods caused additional granule aggregation (Fig. 1). The effect of surface charge and particle size is the primary cause of nanoparticle aggregation. This is because there is a larger concentration of starch suspension present [11]. As a result of the strong acidic state and application heat at 40 °C, acid hydrolysis with hydrochloric acid caused severe disruption and rupture of the granular structure of the starch. The particle appearance of CTSN was significantly disrupted, which increased with processing time as spherical particles were transformed into irregular forms. CTSNs were nanoparticulate in size and smaller than untreated starch (Table 1). CTSN processed in 3 days (558.57 nm ± 38.33) had a smaller granular size than CTSN processed in 6 days (705.07 nm ± 23.89). Despite the fact that, in theory, longer treatment times during acid hydrolysis should be linked with greater size diminution because starch fragments are consecutively liberated from the granule surface, resulting in small particle size [11]. In summary, the particle size of untreated potato starch was greater than that of treated starch in the following order: Untreated potato starch > MTSN-6 > MTSN-3 > CTSN-6 > CTSN-3.

Water absorption capacity (WAC) and solubility of treated and untreated starch

The WAC and solubility of starches are two essential functional characteristics. The treated starches had considerably greater (p < 0.05) WAC, which rose as processing time increased, except for CTSN-3 days (Table 2). The WAC of the treated starches and untreated starch were measured in the following order: CTSN-3 days > CTSN-6 days > MTSN-6 days > MTSN-3 days > untreated starch. WAC is determined by the starch sample's ability to hold water. The large increase in water absorption capacity of CTSN-3 days (94.45 ± 8.22), including other starch nanoparticles, compared to untreated starch (68.09 ± 1.02) can be attributed to a diminution in particle size, that resulted in a rise in surface area of starch nanoparticles formed. The mechanical and chemical hydrolysis of intermolecular connections inside starch granules allowed the hydrogen bonding sites in the granular structure to engage additional water in starch nanoparticles. Ahmad et al. [33] found a similar finding for the WAC of starch nanoparticles made from underused and inexpensive polymer sources in a previous work.

Table 2 Water absorption capacity and solubility of untreated and treated starch (mean ± SD; n = 3)

The modified starches were more soluble than their native form (p < 0.05) (Table 2). However, CTSN-6 days (34.09 ± 5.21) demonstrated more solubility than MTSN-6 days (22.45 ± 5.03). The solubility of the treated and native starches increased in the following order: CTSN-6 days > CTSN-3 days > MTSN-6 days > MTSN-3 days > untreated starch. The increased solubility of CTSN-6 days compared to the other untreated starches may be attributed to the longer period for the hydrolysis of the starch granule, i.e., the starch was exposed to acid treatment for a longer period of time. Because of their broken starch granule, treated or modified starches are soluble in cold water, whereas native or untreated starches have limited water solubility. Native starch can only absorb water in a reversible manner [34]. As previously stated, when a native starch is mechanically or chemically modified, the amorphous area or pattern is disrupted, resulting in a decline in amylose content and a rise in amylopectin content, which enhances the crystallinity of the modified starch. The enhanced WAC and solubility power of treated starches are attributed to the increased amylopectin content and higher concentration of phosphorus groups. The presence of these phosphorus groups on neighboring chains promotes repulsion, resulting in increased hydration due to the weakening of the strong connections inside the amylopectin or crystalline area [35].

Thermal analysis of the untreated and treated starches

Thermal analysis methods including hot-stage microscopy (HSM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were utilized to test the temperature stability (such as isothermal, cooling, or heating) of the UPS and treated starches. These procedures were employed to analyze the physiochemical characteristics of excipients namely melting, evaporation, sublimation, dehydration, phase transitions, oxidation, degradation, and decomposition [36,37,38,39]. HSM was utilized to supplement the results of DSC and TGA.

Differential scanning calorimetry (DSC)

DSC can be employed to determine the transition temperatures of onset [To], midpoint [Tp], conclusion [Tc], and gelatinization enthalpy. These gelatinization characteristics can be used to identify a sample's crystallinity or amorphousness [35].

Figure 2 displays the endotherms and exotherms for untreated and treated potato starches (chemically and mechanically treated starches). The DSC properties of various starches are listed in Table 3. The endothermic peaks observed by all of the starches thermograms at lower temperatures were caused by starch gelatinization [40]. The gelatinization temperatures (To and Tp) increased with chemical (CTSN-3 days-42.23 °C, 79.29 °C) and mechanical (MTSN-3 days-43.89 °C, 85.68 °C and MTSN-6 days-43.06 °C, 85.01 °C) treatments of the native starch (40.40 °C, 76.21 °C) except for CTSN-6 days (39.77 °C, 74.16 °C). According to the DSC results, MTSN-3 days had the greatest gelatinization temperatures, whereas CTSN-6 days had the lowest.

Table 3 Differential scanning calorimetry (DSC) characteristics of untreated potato starch (UPS), chemically treated starch nanoparticle (CTSN), and mechanically treated starch nanoparticle (MTSN)

The rise in Tp of gelatinization of CTSN-3 days after acidic treatment of Ipomoea batatas starch indicates that a considerable amount of the amorphous area of this starch granule was hydrolyzed and changed to crystalline structure or pattern. This finding is consistent with earlier research works [30, 41]. There was a considerable increase in Tp of gelatinization of MTSN-3 days and MTSN-6 days after mechanically modifying sweet potato starch. Mechanical treatment of native sweet potato employing heat, moisture, and shaking for 3 and 6 days reduced amylose concentration, resulting in increased crystallinity of the modified starch granule. Thus, mechanical modification of native starch for 3 days was more successful than 6 days in producing starch nanoparticles. According to the literature, the affiliations of amylose chains within the amorphous area and/or the order of crystallinity in sweet potato starch were altered following heat–moisture–treatment [41, 42].

Endothermic peaks were seen at 220–310 °C before the exothermic peaks in all of the starches. The endothermic peaks were created by the breakdown of long chains in the various starches granules during heating in the DSC. High amylopectin content requires elevated temperature for long-chain breakdown [43]. The temperatures required to break down the long chains in the sample are calculated in Table 3, and they were 45.88 °C, 79.21 °C, 78.39 °C, 66.77 °C, and 33.34 °C for UPS, CTSN-3 days, CTSN-6 days, MTSN-3 days, and MTSN-6 days, respectively. As a result, chemically treated starch nanoparticles had the most amylopectin and would require more energy to break down their lengthy chains. In summary, when compared to the other modified polymers, chemically treated starch nanoparticles processed for 3 days produced the most crystalline nano-carrier.

Exothermic peaks above 320 °C were seen in all samples, indicating starch breakdown. These results corresponded to the maximum temperature of deterioration obtained in thermogravimetric analysis (TGA) curves for all samples. DSC thermograms typically exhibit an exothermic peak due to energy release from the oxidation of the studied sample [43].

Thermogravimetric analysis (TGA)

TGA can be utilized to calculate the sample's percentage mass loss as a result of temperature increase over time. Based on the sample's commencement of degradation temperature, this technique can be used to estimate its thermal stability and decomposition. In addition, it detects moisture and volatile solvents in samples [44,45,46]. TGA curves and thermal events of untreated potato starch and treated potato starches (chemically and mechanically treated starches) processed for 3 and 6 days are shown in Fig. 3 and Table 4. All of the TGA curves demonstrated three significant mass loss portions. For untreated and treated potato starches, the first stage of mass loss begins as soon as the temperature rises (73.75 °C) and finishes at roughly 142.83 °C. This showed that residual solvents were present in all of the samples. At this point, the percentage mass loss is determined by the moisture content of both the treated and untreated potato starches.

Table 4 Thermogravimetric analysis (TGA) thermal events obtained for untreated potato starch (UPS), chemically treated starch nanoparticles (CTSN), and mechanically treated starch nanoparticle

The TGA curves of UPS, CTSN-3 days, CTSN-6 days, MTSN-3 days, and MTSN-6 days (Fig. 3) showed thermal stability until 279.60 °C, 274.19 °C, 263.37 °C, 260.67 °C, and 257.97 °C, respectively. Thus, the order of stability of the native and modified starches was UPS > CTSN-3 days > CTSN-6 days > MTSN-3 days > MTSN-6 days.

The second stage of mass loss is the starch's principal degradation process, which finished at about 370 °C (Table 4). Previous research utilizing TGA revealed that the maximal degradation temperature of native sweet potato starch was 350 °C [40]. This study's findings are supported by the mentioned literature. UPS had the maximum mass loss through degradation at a speed of 10 °C/min over a temperature limit of 279.60–362.35 °C, whereas MTSN-6 days had the lowest mass loss at 63.50% from 257.97 to 356.82 °C. The percentage mass loss by degradation of untreated starch relative to treated starch was in the following order: MTSN-6 days < CTSN-3 days < MTSN-3 days < CTSN-6 days < UPS.

Hot-stage microscopy (HSM)

HSM enables the visual monitoring of changes in a sample's physicochemical characteristics or thermal behavior caused by heating. The thermal behaviors of the samples during TGA and DSC analysis were corroborated by the HSM micrograph in Fig. 4. During the HSM examination of the untreated and treated starches, UPS was shown to be the most stable to heating when compared to the treated starches. UPS > MTSN-3 days > CTSN-3 days > MTSN-6 days > CTSN-6 days was the order of stability. UPS degradation temperature peak on HSM (279 °C) coincided with TGA degradation onset (279.60 °C). The yellowish-brown staining shown in the photomicrographs suggests that the samples were carbonized at temperatures exceeding 260 °C. The ensuing dark-brown hue at the end of the analyses indicated the breakdown of the starches' long chains and small rings [40, 42].

FTIR analysis

FTIR is employed to ascertain the existence of functional groups in a given sample [47]. Figures 5 and 6 displayed the FTIR spectra of untreated and treated potato starches. The FTIR absorption bands of untreated potato starch were compared to those of starch nanoparticles generated using chemical and mechanical methods at two different processing times (Tables 5 and 6). The hydroxyl group, which is a typical peak of native starch, corresponds to the strong broad absorbance band observed at 3268 cm−1 for untreated potato starch (UPS) [48]. The vibration stretching of the methyl and methylene C–H bonds causes the doublet peaks at 2931 cm−1 and 2888 cm−1, respectively [34]. Closely linked water molecules, C–O stretching vibration, and glycosidic linkage (C–O–C) were ascribed to the IR absorption bands at 1637 cm−1, 995 cm−1, and 925 cm−1, respectively [34, 48, 49].

Table 5 Table of untreated potato starch (UPS) and chemically treated starch nanoparticles (CTSN) wavenumbers depicted in FTIR analysis with corresponding functional groupsTable 6 Table of untreated potato starch (UPS) and mechanically treated starch nanoparticles (MTSN) wavenumbers depicted in FTIR analysis with corresponding functional groups

The FTIR spectra of CTSN (Fig. 5) showed reductions in the intensity of distinctive peaks of CTSN-3 days and CTSN-6 days, when compared to UPS. Comparing the native potato starch absorption bands with modified starches, there were small shifts in almost all of the CTSN generated distinctive peaks' IR absorption bands (Table 5), but they have similar functional groups. The vibration stretching of the methylene C–H bond, which was present in UPS (2888 cm−1) was absent in CTSN-3 days, but was present in CTSN-6 days with a somewhat lower band (2884 cm−1).

When compared to UPS, the FTIR spectra of MTSN (Fig. 6) showed declines in the intensity of distinctive peaks of MTSN-3 and MTSN-6 days. Compared to the native potato starch absorption bands, there were minor shifts in almost all of the characteristic peaks of the MTSN generated (Table 6), but all of the characteristic functional groups from the untreated starch were intact. The vibration stretching of the methylene C–H bond, which was seen in UPS (2888 cm−1), was absent in MTSN-3 days but present in MTSN-6 days with a lower band (2880 cm−1).

The reduction in the intensity of characteristic peaks at C–H stretching can be ascribed to changes in the level of amylose and amylopectin content existing in the modified starches [49, 50] The results showed that both acidic and mechanical treatment time on native starch had effects on the amylose and amylopectin content present in starch. Minor changes in the absorption bands of the peak or slight shift in position of the peak between the ranges of 2970 cm−1 to 2910 cm−1 signified the stretching vibration of C–C–H bond [51]. Ipomoea batatas starch usually have high amylopectin content and crystallinity [52]. The slight shift of the absorption band 2931 cm−1 to lower bands at 2928 cm−1 and 2924 cm−1 CTSN 3 days and CTSN 6 days respectively and also for MTSN 3 days and MTSN 6 days at 2928 cm−1, signifies the increase in amylopectin content, thus leading to increase in the crystallinity of the modified starches. From a previous study, observable reduction in the peak intensity at 1635 cm−1 and 1637 cm−1 for CTSN 3 days and MTSN 3 days respectively (Tables 5 and 6) compared to USP (1637 cm−1) means that as the crystallinity of two chemically and mechanically treated starch for 3 days increases, the absorption bands become weaker [52].

The decrease in the intensity of distinctive peaks during C–H stretching can be attributed to changes in the amylose and amylopectin content of the modified starches [49, 50]. The results demonstrated that the amylose and amylopectin concentration of native starch was affected by both acidic and mechanical treatment duration. Minor variations in the peak's absorption bands or a slight shift in peak position between 2970 and 2910 cm−1 indicated the stretching vibration of the C–C–H bond [51]. Sweet potato starch is often high in amylopectin and crystallinity [52]. The slight shift of the absorption band 2931 cm−1 to lower bands at 2928 cm−1 and 2924 cm−1 for CTSN-3 days and CTSN-6 days, respectively, as well as for MTSN-3 days and MTSN-6 days at 2928 cm−1, indicates a rise in amylopectin content, which leads to intensification in crystallinity of the modified starches. According to a previous study, the observable decrease in peak intensity at 1635 cm−1 and 1637 cm−1 for CTSN-3 days and MTSN-3 days, respectively (Tables 5 and 6) compared to USP (1637 cm−1) means that as the crystallinity of two chemically and mechanically treated starches increases for 3 days, the absorption bands become weaker [52].

RAMAN characterization

The structural fingerprints obtained by Raman spectroscopy can be used to identify samples [52, 53]. Raman is typically used to supplement FTIR data [38]. In some circumstances, Raman will detect vibration patterns that FTIR would not detect, and vice versa [54, 55]. Figures 7 and 8 show the Raman spectra of untreated and treated potato starches. Tables 7 and 8 display the similarities and differences in the characteristic bands of UPS and the treated starches. The UPS spectra revealed prominent peaks at 472 cm−1, 859 cm−1, 930 cm−1, 1078 cm−1, 1249 cm−1, 1341 cm−1, and 1456 cm−1, which aided in the identification of the untreated starch. Skeletal modes, CH and CH2 deformation, bending vibration of C–O–H, and C–O–H bending were given to characteristic bands at 472 cm−1, 859 cm−1, 930 cm−1, and 1078 cm−1, respectively. Furthermore, significant absorption bands at 1249 cm−1, 1341 cm−1, and 1456 cm−1 were associated with the CH2 OH-related mode functional group, vibration CH2 twisting and bending of C–O–H, and CH2 deformation, respectively. These characteristic peaks are similar to those described in the literature [50]. The CTSN spectra (Fig. 7) demonstrated that it kept almost all of the UPS peaks with just negligible changes in peak strength. MTSN spectra (Fig. 8) revealed that UPS major peaks were kept with nearly the same intensity count, but the distinctive peak at 1078 cm−1 was missing in both MTSN-3 and MTSN-6 days.

Table 7 Untreated potato starch (UPS) wavenumbers portrayed in Raman analysis with corresponding functional groups compared to chemically treated starch nanoparticles (CTSN-3 days and CTSN-6 days) absorption spectrumTable 8 Untreated potato starch (UPS) wavenumbers portrayed in Raman analysis with corresponding functional groups compared to mechanically treated starch nanoparticles (MTSN-3 days and MTSN-6 days) absorption spectrumIn vitro release profile of the ciprofloxacin-loaded starch nanoparticles

In comparison with the untreated starch (Fig. 9 and Table 9), release times were significantly longer for the chemically and mechanically modified starches in the rank order of t80, CTSN-3 > MTSN-3 >  > CTSN-6 > MTSN-6 >  >  > untreated starch. The untreated starch demonstrated immediate release, where 80% of drug was achieved in 61 min.

Table 9 Release times of ciprofloxacin-loaded untreated potato starch and starch nanoparticles

All the nanoformulations, irrespective of method and processing times, and the untreated starch demonstrated release properties which fitted the Korsmeyer–Peppas model (Qt/Qa = kktn) (Table 10). The model is typically known as “Power law,” and it describes the drug dissolution from a polymeric system such as starch used in this study. The kinetics of drug release was indicated by the constant n in Korsmeyer–Peppas model (Qt∕Qa = kktn). The constant n is the diffusional exponent or release exponent, indicative of the drug release mechanism. All the nanoformulations demonstrated n < 0.5, while the untreated starch had n > 0.5 < 1.0. When 0.5 < n < 1.0, it indicates anomalous (non-Fickian) diffusion. The nanoformulations and the untreated starch thus had release which followed non-Fickian diffusion [56].

Table 10 Correlation coefficients (R2) obtained for ciprofloxacin-loaded nanoparticles