The design and evaluation of ciprofloxacin-loaded nanoformulations using Ipomoea batatas starch nanoparticles

Abstract

Background

Starch nanoparticle derivatives are gaining popularity as drug delivery vehicles because of their biocompatibility, better mechanical characteristics, heat stability properties, impediment qualities, permeability capabilities, and flexibility to be changed for specific predetermined functions. The effect of techniques and processing time on the physiochemical and drug release characteristics of sweet potato (Ipomoea batatas) starch nanoparticles and their ciprofloxacin-loaded nanoformulations was studied.

Results

Scanning electron microscopy confirmed that the treated starch formed nanoparticles and also revealed significant changes in the morphology of the treated starches. The water absorption capacity of chemically treated starch nanoparticles (CTSN)-3 days was the highest, whereas CTSN-6 days had the maximum solubility. The functional groups present in the starch nanoparticles were confirmed by Fourier transform infrared spectroscopy and Raman. The thermal characteristics of starch nanoparticles were established using hot-stage microscopy, differential scanning calorimetry, and thermogravimetric analysis. The percentage drug content and loading efficiency of the model drug were extensively boosted by the chemical and mechanical treatment of Ipomoea batatas starch. In comparison with the untreated potato starch (UPS), release times for loaded drug were significantly longer for the chemically treated starch nanoparticles and mechanically treated starch nanoparticles (MTSN) starches in the rank order of T_80%, CTSN-3 days > MTSN-3 days > CTSN-6 days > MTSN-6 days > UPS. The main kinetics of drug release were Fickian diffusion.

Conclusion

After 3 days of acid hydrolysis, sweet potato starch yielded nanoparticulate carriers that can be employed for controlled or extended release of medicines that are poorly water soluble.

Background

Nanotechnology is the design and fabrication of systems at the atomic and molecular levels in order to manufacture objects as small as a few hundred nanometers. These systems are created by building each component from the top-down or bottom-up. The usage of nanotechnology in the delivery of drugs has altered the landscape of pharmaceutical and biotechnology firms [1,2,3,4]. Nanoshells, polymer conjugates, nanoparticles, dendrimers, micelles, carbon nanotubes, proteins, liposomes, and quantum dots are some examples of nanotechnology drug delivery methods used [1, 4].

Nanoparticles are colloidal particles with diameters spanning from 1 to 1000 nm; although several preferential sizes exist, they are employed for specific nanomedicine applications [1]. Nanoparticles manufactured from natural polymers have gained significant interest because (1) they can be modified for targeted drug administration; (2) they improve solubility and bioavailability; (3) they serve as important carriers in the delivery of poorly soluble pharmaceuticals; and (4) they can control drug release from a single dose and prevent endogenous enzymes from degrading it [5].

Starch is a biopolymer that is naturally occurring, plentiful, and renewable. Its low cost, biodegradability, non-toxicity, and biocompatibility make it particularly appealing and of great interest in numerous nanoparticulate research fields [6, 7]. Many plants store starch as an energy source during photosynthesis. This polymer has glucose-based amylose and amylopectin granules. Starch is derived from a variety of plant types, including cereals or grains, roots, tubers, legumes, and fruits [4, 8, 9]. Because of its granules' poor solubility, weak reactive surface, high viscosity, and ability to create a cohesive paste, starch has limited utility in industry. Modifications can improve certain qualities and add more functional groups to starch [4, 10, 11].

Starch nanoparticles (SNPs) are particles with at least one dimension less than 1000 nm but greater than a single molecule or atom [10]. Starch nanoparticles are not the same as starch nanocrystals. Starch nanocrystals, also known as starch crystallite or microcrystallite, are crystalline or formed from the disruption of the amorphous domain of the semicrystalline structure of starch granules, whereas starch nanoparticles are amorphous and formed from gelatinized starch [12, 13].

Gallant et al. [14] and Tang et al. [15] found nano-sized semicrystalline blocklets in starch granules. Mild hydrolysis with acids or enzymes can extract nano-blocklets from starch. Physical therapy may potentially break down the starch granules, allowing the nano-blocklets to be released. These SNPs contain a crystalline moiety and the inherent advantages of starch granules, namely renewability and biodegradability [16]. SNPs can be produced through different types of methods, including chemical (acid hydrolysis), mechanical/physical (sonication, milling, thermosonication, spray-drying), enzymatic (enzyme debranching), and nano-precipitation, which convert the morphology of starch granules to nanoparticles [4, 6, 10]. When compared to native starch, starch nanoparticles exhibit distinct physiochemical and biological properties. This potential property includes better solubility, improved biological penetration rate, strong reactive surfaces, and increased absorption capacity, allowing them to be used as carriers and for the transportation of bioactive substances into impermeable anatomical regions instead of lipid micelles [10].

Ipomoea batatas (sweet potato) (Convolvulaceae family) is a perennial plant found in many warm temperate, subtropical, and tropical climates. Sweet potatoes are a high-starch food and medicinal ingredient. Sweet potato starch concentration ranges from 6 to 31%. Previous research has used Ipomoea batatas starch as a binder and disintegrant [17, 18]. Sweet potato was selected for the study for the following reasons. Potato is reported as one of the world's most versatile and under-exploited food crops with high nutritional value. The starch is preferred in the food and pharmaceutical industry because the paste has acceptable clarity due to the low amount of protein and lipids; in addition, the flavor is neutral unlike other starches [19].

Ciprofloxacin inhibits Gram-positive and Gram-negative bacteria. It treats lung, urinary, otitis media, and external ocular infections [20, 21]. Ciprofloxacin is classified as class 4 by the Biopharmaceutical Classification System, signifying low permeability and solubility, resulting in poor bioavailability as well as poor absorption and considerable variability [22]. Ciprofloxacin is available in tablet form and is administered twice a day at a dose of 500 mg. The use of starch nanoparticles in the delivery of ciprofloxacin is predicted to improve the drug's solubility, bioavailability, and cellular permeability, resulting in a dose or frequency of administration reduction. The goal of this work was to determine how different preparation methods and processing times affected the characteristics of Ipomoea batatas starch nanoparticles and their ciprofloxacin-loaded nanoformulations.

Materials and methods

Materials

Ipomoea batatas was sourced locally. The potato starch was prepared in the laboratory.

Methods

Extraction of Ipomoea batatas starch

Ipomoea batatas tubers were washed with distilled water, peeled, and wet-milled using a laboratory blender (Panasonic Mixer and Grinder MX-AC400). The slurry was sieved using a calico cloth to remove the chaff, and the filtrate was allowed to settle before being decanted. The starch sediment was constantly washed until the supernatant was colorless. The wet bulk was dried in a hot air oven (Gallenkamp BS Oven 250) at 50 °C for 48 h. The dried potato starch was mixed in a laboratory mill and screened through a 120 mesh sieve to produce a fine powder that was weighed and kept in sealed containers until needed [23].

Preparation of starch nanoparticles

Chemically treated starch nanoparticles (CTSN) SNPs were synthesized using the procedures published by Kim et al. [24], with minor modifications. 800 mL of 2.2 M hydrochloric acid was used to hydrolyze 40 g of untreated potato starch (UPS). A temperature-controlled mechanical shaker was used to continually mix the suspension at 100 rpm at 40 °C. (Brunswick Scientific Incubator Shaker, Thermo Scientific, USA). Two batches were prepared by varying the incubation period for 3 and 6 days, respectively. After the experiments, successive washings were done to return the starch to its initial pH of 4. Following that, the starch nanoparticles were freeze-dried (Christ/Alpha 1–2 LD plus Dryer). For the mechanically treated starch nanoparticles (MTSN), the UPS suspensions were subjected to mechanical shaking and stirring at 100 rpm at 40 °C for 3 and 6 days, respectively, and then freeze-dried.

Water absorption capacity and solubility (WAC)

WAC was determined by adding distilled water (15 ml) to 2.5 g of USP, CTSN, and MTSN. The samples were vortexed for 2 min before being centrifuged at 400 rpm for 20 min. The supernatant was decanted, and the residue was weighed and dried to a constant weight in an oven. The WAC was then expressed as the weight of water bound to 100 g of each sample. Similarly, solubility was determined with 1 g (W) of sample in a conical flask and 15 ml of distilled water was added and agitated for 5 min. It was placed in a water bath and heated at 60 degrees for 20 min with constant stirring. It was put into a pre-weighed centrifuge tube (W1) and centrifuged for 20 min at 2200 rpm. The supernatant was decanted into a pre-weighed dish (W2), dried at 100 degrees to constant weight (W3), and cooled [25].

$${\text{Solubility}}\left( \% \right) = \left\{ {\left( {W{2} - W{3}} \right)/W} \right\} \times {1}00$$

Preparation of ciprofloxacin-loaded starch nanoparticles

Ciprofloxacin (250 mg) was dissolved in 250 ml acetic acid (0.1 M). The treated or untreated potato starch (5 g) was then dispersed into the prepared solution of ciprofloxacin. The mixture was placed on a magnetic stirrer and rotated for 1 h. The mixture was centrifuged, decanted, and air-dried. A UV spectrophotometer was used to measure the quantity of drug in the supernatant. The following formulae were used to compute the drug content and loading efficiency [13]:

$${\text{Drug}}\;{\text{content}}\;\left( \% \right) = \left( {x/y} \right) \times {1}00$$

where x = weight of the drug in the starch nanoparticle; y = weight of the starch nanoparticle.

$${\text{Loading}}\;{\text{efficiency}}\;\left( \% \right) = \left( {a/b} \right)*{1}00$$

where a = weight of the drug in the starch nanoparticle; b = first weight of the drug in the loading solution.

Characterization of untreated potato starch (UPS), chemically treated starch nanoparticles (CTSN), and mechanically treated starch nanoparticles (MTSN)

Scanning electron microscopy (SEM)

The surface structure and particle size of the UPS and SNPs were determined utilizing SEM (JEOL, JSM-6060LV (Tokyo, Japan). Samples were placed on aluminum stubs and glazed with a gold film under a vacuum in a sputter coater. The observation was done at different magnifications.

Differential scanning calorimetry (DSC)

The samples' DSC thermograms were generated using a PerkinElmer DSC 8000. (Waltham, USA). These samples were prepared by precisely weighing around 2.5 g of each sample into an aluminum pan and completely closing it with an aluminum cover. Following that, heating from 25 °C (room temperature) to 400 °C at a pace of 10 °C/min in an inert nitrogen environment with a flow tempo of 20 ml/min. The reference was a covered, empty aluminum pan [26].

Thermogravimetric analysis (TGA)

The samples' TGA thermograms were determined using a Perkin Elmer TGA 4000. (Waltham, USA). In a porcelain crucible, about 3 mg sample was deposited. The sample was then heated from 30 °C up until 600 °C with a nitrogen purge of 20 ml/min and a heating speed of 10 °C/min [26].

Hot-stage microscopy (HSM)

The samples' HSM micrographs were obtained utilizing an Olympus optical (Japan) microscope with a LinkPad THMS600 heating stage (Linkam Scientific Instruments Ltd., Surrey, UK). Each sample was soaked in silicon oil and heated from ambient temperature (25 °C ± 2) at a speed of 10 °C/min until degradation was noticed. Using an Olympus UC30 camera and Stream Essentials software linked to the microscope, images were captured and temperatures were recorded at each detectable thermal change [26].

Fourier transform infrared spectroscopy (FTIR)

The samples' FTIR spectrograms were determined utilizing a Perkin Elmer Spectrum 400 spectrometer (Waltham, USA). The samples were placed on the equipment's already cleaned diamond crystal. For each sample, the analysis was performed at 650–4000 cm⁻¹ with four scans at resolutions ranging from 2 to 4 cm⁻¹/s [26].

Raman spectroscopy

The samples' Raman spectrograms were produced using an Anton Paar Raman spectrophotometer. The samples were set in the equipment sampling chamber after being poured into small conical glass vials. The data were analyzed using a 1064 nm wavelength in the range of 0 to 2500 cm⁻¹.

In vitro release study of Ciprofloxacin-loaded starch nanoparticles

The release measurement in 900 ml buffer solution was conducted in a dissolution test apparatus using phosphate buffer of pH 6.8. The drug-loaded starch nanoparticles and drug-loaded untreated potato starch were packed into capsule shells. The buffer medium in the receptor vessels was maintained at 37 °C and rotated at 100 rpm. Approximately 5 ml of the receptor vessels’ fluid were withdrawn at various predetermined durations (5, 10, 15, 30, 45, 60, 90, 120, and 150 min), and the percentage of the extract-mediated silver nanoparticles released was analyzed using UV–visible spectrophotometer at 315 nm. The sink method was maintained, through the replacement of 5 ml of the pure buffer solution after every sample withdraws from the vessels. The cumulative percentage drug released was calculated and plotted versus time [27].

Results

Figures 1, 2, 3, 4, 5, 6, 7, 8 and 9.

Fig. 1

SEM micrographs of a untreated potato starch, b chemically treated potato starch-6 days, and c mechanically treated potato starch-6 days. Scanning electron microscopy (SEM) photomicrographs of untreated potato starch, chemically treated starch nanoparticles-6 days, and mechanically treated starch nanoparticles-6 days

Fig. 2

The DSC thermograms for UPS, CTSN-3 days, CTSN-6 days, MTSN-3 days, and MTSN-6 days obtained during heating ranging from 25 to 400 °C at a heating speed of 10 °C/min. The evaluation of the differential scanning calorimetry (DSC) thermogram of untreated potato starch (UPS) versus chemically treated starch nanoparticle (CTSN) and mechanically treated starch nanoparticle (MTSN)

Fig. 3

The overlay of TGA curves obtained for UPS, CTSN-3 days, CTSN-6 days, MTSN-3 days, and MTSN-6 days during heating at a rate of 10 °C/min from 30 to 600 °C. The comparison of thermogravimetric analysis (TGA) curve of untreated potato starch (UPS) against chemically treated starch nanoparticle (CTSN) and mechanically treated starch nanoparticle (MTSN)

Fig. 4

Photomicrographs illustrating the thermal behavior of UPS, CTSN-3 days, CTSN-6 days, MTSN-3 days, and MTSN-6 days during heating at a rate of 10 °C/min. The overlay and comparison of hot-stage microscopy pictograms of untreated potato starch (UPS) versus chemically treated starch nanoparticles (CTSN) and mechanically treated starch nanoparticles (MTSN)

Fig. 5

The overlay of FTIR spectra of UPS, CTSN-3 days, and CTSN-6 days. The comparison of the Fourier transform infrared spectrum of untreated potato starch (UPS) against chemically treated starch nanoparticles (CTSN)

Fig. 6

The overlay of FTIR spectra of UPS, MTSN-3 days, and MTSN-6 days. The comparison of the Fourier transform infrared spectrum of untreated potato starch (UPS) against mechanically treated starch nanoparticles (MTSN)

Fig. 7

The Raman spectra overlay of untreated starch and chemically treated starches. The comparison of the Raman spectrum of untreated potato starch (UPS) against chemically treated starch nanoparticles (CTSN)

Fig. 8

The Raman spectra overlay of untreated starch and mechanically treated starches. The comparison of the Raman spectrum of untreated potato starch (UPS) versus mechanically treated starch nanoparticles (MTSN)

Fig. 9

Release profiles of ciprofloxacin-loaded chemically treated starch nanoparticles, mechanically treated starch nanoparticles, and untreated potato starch. The release study was conducted over a duration of 400 min

Discussion

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 [T_o], midpoint [T_p], conclusion [T_c], 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 (T_o and T_p) 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 T_p 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 T_p 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 groups

Table 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 spectrum

Table 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 spectrum

In 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

Conclusion

The characteristics of the produced starch nanoparticles were regulated by techniques and processing time. Particle sizes rose as processing time increased, which was not a good thing. Chemically created starch nanoparticles had longer release times than mechanically prepared starch nanoparticles, and shorter processing times appeared to be more favorable in both preparation processes. Chemically treated starch nanoparticles for 3 days demonstrated the smallest particle sizes, maximum drug content, loading efficiency, and percentage water absorption capacity; it can thus be enhanced as a ciprofloxacin carrier. These findings demonstrated that varied processing methods and time can alter the features of nanoparticles created from a specific polymer, and they will aid in the development of recommendations for developing future nanoparticles.