PhD Thesis

Synthesis, Characterization and Application of Graphene Based Materials

Tue, 04 Aug 2026 00:00:00 GMT

Synthesis, Characterization and Application of Graphene Based Materials Hoque, Mohammad Amirul Graphene is being exponentially utilized across diverse research fields due to its extraordinary ability to enhance material properties with its miraculous attributes. Still, the material is expensive due to difficulties in the production of high-quality graphene. In this study, a few initiatives were approached to reduce the production costs of graphene with improved properties. At first, Tour's modified Hummer’s method was adopted to synthesize improved graphene with the feed acid liquor (FAL) recycling technique. About 90% of the FAL were recycled and reused five times as feed for successive production batches. Another focus was on the oxidation reaction to achieve graphene oxides (GOs) with higher oxygen to carbon ratios. The changes in recycled FAL and synthesized GOs properties due to repeated recycling were evaluated. The synthesized GOs were utilized for the removal of arsenic (As3+) ions from water, showing a qmax of 343.14 mg/g with 98.4% removal efficiency from 300 ppm to 22 ppm. The GOs were then reduced thermally at various temperatures in water, kerosene, and kerosene followed by ascorbic acid methods and times. The major objective of the reduction was to achieve maximum reduction of GO. The synthesized reduced graphene oxides (rGO) were evaluated for the antimicrobial properties. Although the Hummer’s method produces GO and rGO with improved properties, the method is difficult to carry out, very slow, and expensive. To obtain an easy and faster method for the production of graphene in bulk, a microwave assisted rapid exfoliation process of graphene exfoliation was examined along with the effect of variation of the intercalating agent ratio. All the products of FAL, GOs, rGOs, and microwave graphene (MG) were characterized using a Karl-Fischer moisture analyzer, IC, AAS, FT-IR (ATR), FT-Raman, UV-Visible Spectroscopy, PS-Zeta potential, XPS, XRD, STA, TGA, FE-SEM, and TEM analyzer. The FAL had increased moisture content with the successive recycling, but had no significant changes in properties and composition of GO. However, GO properties greatly changed with temperature and time of reaction, and the amount of oxidizing agent. The degree of reduction of GOs in N2 atmosphere at 193.4 oC was 95% of its initial mass, even at a slow rate of heating, but at this temperature, GO explosively degraded. In water medium as well as in kerosene, the explosive degradation can be omitted, and even at 220 oC, affording 40% reduction, which was mainly due to the removal of oxygen atoms as evident from the XPS analysis. The extremely reduced rGO contained an oxygen-to-carbon ratio of only 2.49% based on its initial oxygen content. In this study, XRD data also supported the composition of rGO and showed that it contained a homogeneous amorphous or viii nanocrystalline structure, as demonstrated by the 2θ = 24.91° peak in the (002) plane, which was shifted from the GO peak at 2θ = 12.86° in the (001) plane. The SEM image measured by ImageJ software showed that the obtained grain sizes were between 100 and 200 nm for both the GO and rGO. These rGO also showed strong activities against gram-positive and gram-negative bacteria, such as B. subtilis, S. aureus, E. coli, and S. typhi. In the case of MG, the degree of exfoliation was directly proportional to the intercalating agent ratios, and this method directly produces pristine graphene. This MG was highly thermo-stable up to 700-800 oC with high crystallinity, having 2θ = 26.56° at the (002) plane. The crystalline graphene showed excellent adsorption of acid blue-25 dyes with qmax 472.8 mg/g and 94.56% removal efficiency in the case of a 300-ppm dye solution. This thesis is submitted for the degree of Doctor of Philosophy

Superiority Enhancement of Geo-Jute by Chemical Modification Through Gamma Radiation

Mon, 03 Aug 2026 00:00:00 GMT

Superiority Enhancement of Geo-Jute by Chemical Modification Through Gamma Radiation Akter, Nasrin Natural fiber jute inherently possesses greater tensile strength (TS) than other natural fibers and offers numerous applications in geotechnical applications as a replacement for synthetic fibers. This research aims to enhance the physio-mechanical properties of jute fabric by improving its environmental durability. The study improves durability by chemically modifying materials using gamma radiation. Bitumen emulsion and unsaturated polyester resin were used for chemical modification. First, the jute fabric is treated with a chemical (polymer mixture), and a suitable formulation (30% BE + 10% UPR) is chosen based on durability tests. Then, the chemically treated jute fabric is irradiated with gamma radiation, selecting an appropriate dose (5 kGy) through durability testing. Finally, the pre-treated jute fabric is subjected to chemical treatment and gamma radiation, and its durability is assessed using an accelerated weathering tester. Pre-treatment was performed in two ways: (i) HEMA pre-treated, and (ii) benzoyl peroxide pre-treated. The HEMAtreated jute fabric showed better durability than the other treated sample. The tensile strength of HEMA-pre-treated samples was 50% higher than that of the raw jute fabric. At each step, samples were characterized by FT-IR, XRD, and TGA analyses, and their physico-mechanical properties were measured through tensile strength and water absorption tests. After chemical modification, the OH groups in the jute fabrics are reduced, decreasing hydrophilicity and increasing hydrophobicity, thereby enhancing the physico-mechanical properties. This research aims to strengthen these properties without making the fabric entirely plastic by breaking additional OH bonds. Pre-treatment generates free radicals from jute cellulose, and gamma radiation promotes their formation and cross-linking within the cellulose-polymer matrix. Cross-linking improves the fabric's physico-mechanical properties. Durability and physical tests show improvements in these properties. FTIR, XRD, and TGA analyses confirm the formation of free radicals and the crosslinking of jute cellulose with the polymer mixture. This research partially enhances the hydrophobicity of jute fabric. Gamma radiation not only boosts cross-linking but also reduces excess chemicals on the fabric surface, increasing porosity between yarns and making the fabric more biodegradable and eco-friendlier. An accelerated weathering test compares durability under artificial and outdoor conditions. In the lab, an accelerated weathering tester provides quicker results than outdoor exposure by controlling temperature, light, and water spray. Overall, durability testing using this method yields faster results than natural outdoor testing. Significant improvements in TS and durability testing were observed in the treated jute fabric compared to the 4 raw jute fabric. Whereas raw jute fabric deteriorates after 45 days, treated jute fabric showed enough tensile strength after 90 days. On the other hand, in the accelerated weathering durability test, after 28 days, the treated jute fabric showed significantly higher tensile strength and weight than the raw jute fabric. This thesis is submitted for the degree of Doctor of Philosophy.

Isolation and Characterization of Amylase Producing Microbes and Its Sequential Strain Development for Industrial Enzyme Production

Mon, 02 Mar 2026 00:00:00 GMT

Isolation and Characterization of Amylase Producing Microbes and Its Sequential Strain Development for Industrial Enzyme Production Fuadh-Al-Kabir, Md. Amylase enzymes are essential biocatalysts with wide-ranging industrial applications, including food processing, textile desizing, detergent formulation, paper production, and pharmaceuticals, owing to their ability to hydrolyze starch into simpler sugars. Despite the rising industrial demand, Bangladesh currently lacks domestic amylase production facilities, resulting in a dependence on total imports. To address this challenge, the present study aimed to isolate potent amylase-producing bacterial strains from natural sources, followed by their characterization, strain improvement through mutagenesis, optimization of production parameters, and evaluation of their industrial applicability. The ultimate goal was to develop a sustainable and indigenous source of amylase for local utilization. Following the harvesting period, soil samples were obtained from potato-growing areas in the Munshiganj and Rangpur districts, specifically from sites where remnants of decomposing potato matter remained in the fields. A total of 128 microbial isolates comprising 69 bacterial and 59 fungal strains were obtained using nutrient agar and potato dextrose agar media. Primary screening for amylase activity was conducted via starch agar assays followed by iodine staining, and hydrolysis zone-to-colony diameter ratios were determined. Among the bacterial isolates, S-1 and S-2 showed the highest hydrolytic potential, with the zone ratios of 3.44 and 4.98, respectively. Secondary screening using the dinitro salicylic acid (DNS) method revealed that isolate S-2 exhibited the highest crude amylase activity (7.98 ± 0.26 U/mL), compared to S-1 (5.21 ± 0.21 U/mL). Based on these findings, isolate S-2 was selected for further investigation. Protein concentration estimated by the Folin–Lowry method yielded 0.981 ± 0.03 mg/mL, with a corresponding specific activity of 8.14 ± 0.25 U/mg, confirming the isolates for industrial potential. Morphological analyses (Gram staining, spore staining, colony traits) indicated that isolate S-2 belonged to the Bacillus genus. This was supported by biochemical tests including catalase, Simmons’ citrate, starch hydrolysis, nitrate reduction, Voges-Proskauer and indole tests. The bacterial isolate was conclusively identified as Bacillus subtilis based on comprehensive molecular analysis involving 16S rRNA gene amplification, nucleotide sequencing, and subsequent phylogenetic evaluation showing over 99% similarity with reference sequences from the NCBI database. Amylase production from Bacillus subtilis S-2 was significantly enhanced through systematic strain improvement using physical (UV, gamma) and chemical (EMS) mutagenesis. The wild-type strain exhibited an initial amylase activity of 7.98 ± 0.26 U/mL, ii which significantly increased following mutagenic treatment. Among the conditions tested, UV exposure at 254 nm for 15 minutes resulted in the highest enzyme activity of 15.72 ± 0.32 U/mL, representing a 96.99% enhancement compared to the wild type. Gamma irradiation (1.0 kGy) and EMS (0.5%, 60 min) also significantly boosted production (15.11 ± 0.34 and 15.22 ± 0.48 U/mL, respectively). The UV-induced mutant (S2-UV3) was selected for further optimization. To maximize enzyme production, a dual-stage optimization strategy was employed, initially utilizing the conventional one-variable-at-a-time (OVAT) technique, followed by a more refined statistical optimization through Response Surface Methodology (RSM) based on the Box–Behnken Design (BBD). OVAT revealed optimal conditions pH 7.0, temperature at 45°C, 72 hours incubation and 1.5% starch achieving 24.21 ± 0.52 U/mL activity. Peptone (1.0%) was the best nitrogen source. Both commercial soluble starch and locally sourced potato starch yielded comparable enzyme activity (~24 U/mL), indicating agro-industrial applicability. RSM optimized the process further, with a robust model (R² = 98.96%, p < 0.0001). Predicted conditions (pH 7.02, 46.06°C and 1.56% starch) yielded 26.12± 0.46 U/mL in validation trials. Scaling-up in a 5 L fermenter with 1 vvm aeration and 130 rpm agitation improved production by 19.15%, reaching at 31.12± 0.62 U/mL attributed to enhanced oxygen transfer and temperature control. These results confirm process scalability and economic viability using locally available resources. Purification of amylase from UV-mutated (S2-UV3) was performed from RSM-optimized cultures. The preliminary crude enzyme preparation demonstrated an activity level of 26.12 ± 0.46 U/mL, accompanied by a protein content of 0.98 ± 0.04 mg/mL, thereby yielding a specific enzymatic activity of 26.65 U/mg protein. Following a three-step purification protocol comprising ammonium sulfate precipitation, dialysis, and gel filtration chromatography, the enzyme was recovered with an activity of 64.22 U/mL and a significantly enhanced specific activity of 133.79 U/mg. This process resulted in a 5.02-fold purification and a yield of 74.8%, indicating the effectiveness and reliability of the purification protocol in concentrating and refining the target enzyme. For long-term application, S2-UV3 was evaluated for strain preservation. Cryopreservation at –80°C in 20% glycerol retained ~80% activity after 12 months, while 4°C storage showed notable decline after 6 months. Thus, –80°C with glycerol is the optimal condition for long-term storage. Purified enzyme stability was assessed under varied storage conditions. While 4°C was maintained >70% (47.44 U/mL) activity for 6 months, significant losses occurred. Storage at iii –20°C and –80°C retained 75–78% activity after a year. Inclusion of 0.1% sodium azide and 20% glycerol enhanced stability up to 87%, while lyophilized enzyme at 4°C preserved over 92% activity. The lyophilization and freezing with glycerol are the most effective strategies for long-term enzyme stability. SDS-PAGE of the purified enzyme showed a single band (~56 kDa), confirming molecular purity and alignment with known Bacillus α-amylases. Absence of additional bands indicated structural homogeneity, essential for consistent industrial use. The purified amylase (64.22 U/mL) showed strong industrial applicability. In textile desizing, complete starch removal from cotton fabric was confirmed via iodine staining and reducing sugar release (4.20 ± 0.11 mg/mL). In detergent-assisted cleaning, enzyme-detergent synergy yielded maximum starch stain removal (4.86 ± 0.15 mg/mL). These findings validate the enzyme's eco-friendly, effective role in textile and detergent industries. In conclusion, this study successfully established a comprehensive work for the isolation, characterization, and mutagenesis-based improvement of a potent amylase-producing Bacillus subtilis strain sourced from locally collected soil samples. The use of low-cost agricultural waste, such as potato peels, as an alternative carbon source combined with systematic optimization and multistep purification led to the development of a highly active and stable enzyme. The purified amylase exhibited excellent performance in eco-friendly industrial applications, including textile desizing and detergent-based starch stain removal. Altogether, the optimized production process and improved bacterial strain offer a valuable biotechnological resource for sustainable, indigenous enzyme production in Bangladesh and hold promising potential for future industrial scale-up and commercialization. This thesis is submitted for the degree of Doctor of Philosophy.

Studies on green jute pulping processes

Tue, 27 May 2025 00:00:00 GMT

Studies on green jute pulping processes ALAM, MUHAMMAD RABIUL This thesis is submitted for the degree of Doctor of Philosophy.