|Nur Halilatul Sadiqin Omar Ali||Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Siti Norliza Sa’adah Abdul Rahman||Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Hajar Azirah Adol||Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Malai Haniti Sheikh Abdul Hamid|
|Hussein Taha||Environmental and Life Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Mohd Yameany Haji Rosli||Geosciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Narayana T. R. Nilantha Kumara||Centre for Advanced Material and Energy Sciences, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
|Abdul Hanif Mahadi||Centre for Advanced Material and Energy Sciences, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam|
this study, the microcrystallization of the 2-thiophenecarboxaldehyde Schiff
base of S-benzyldithiocarbazate (i.e.
[TASBnDTC]) was fabricated by a reprecipitation method in an organic
solvent-water system using different crystallization parameters, including
temperature and the concentration of the target compound. The size, anisotropy,
crystalline phase, and surface morphology of the TASBnDTC microcrystals were
characterized by dynamic light scattering and scanning electron microscopy. The
stability of the Schiff base microcrystals was also evaluated. Different sizes
of surfactant-dispersed TASBnDTC microcrystals (1505, 2194, and 2447 nm) were
fabricated from three different concentrations of the Schiff base (0.001 M,
0.002 M, and 0.003 M, respectively) in an acetone-water system. The TASBnDTC
microcrystals were also evaluated by X-ray powder diffraction and
were found to differ slightly in molecular form but were otherwise similar,
irrespective of the different TASBnDTC concentrations. The synthesized Schiff
bases and their microcrystals were also screened for their antibacterial
activities against four different gram-positive and gram-negative bacterial
strains (Escherichia coli,
Bacillus subtilis, Pseudomonas aeruginosa, and Staphylococcus
aureus) using the agar well
diffusion method. The growth inhibition was
enhanced by 8.0 to 10.75 mm against the four bacteria by TASBnDTC microcrystals
compared to the bulk molecular form, which showed no inhibitory activity at
all. However, the inhibition was less that that achieved with the standard
streptomycin antibiotic, which gave zones of inhibition of 18.0 to 23.0 mm
against the four bacterial strains. Overall, the Schiff base microcrystals show
potential for use in various biological applications. They also have potential
physical and optical applications due to their high surface-to-volume ratio and
the molecular alignment on the surface of the microcrystals.
Antibacterial activity; Nanoscience; Organic microcrystal; Reprecipitation method; Schiff base
In recent times, nanotechnology has been integrated in many parts of our everyday lives. The fabrication of organic and inorganic nano- and microcrystals is one area of growing interest from the perspectives of both fundamental science and applications (Poerwadi et al., 2020; Strokova et al., 2020). However, organic microcrystals have received little attention so far because of difficulties with their characterization and the thermal instability of most of the organic compounds.
Organic microcrystals with sizes of several tens of nanometers to the sub-micrometer crystal size have been fabricated by either “top-down” approaches, such as milling methods (Li et al., 2020), or by “bottom-up” approaches, such as reprecipitation (Kasai et al., 1992; 2012) and supercritical fluid crystallization (SCFC) (Komai et al., 1999). In the “top-down” approaches, the microcrystals are fabricated by size reduction techniques, whereas in the “bottom-up” approaches, the microcrystals are fabricated from atoms or molecules by synthetic chemistry or self-assembly processes. The most common “bottom-up” approach is the so-called reprecipitation method first reported by Kasai et al. (1992). The reprecipitation method has many advantages, as it is simple, quick, and inexpensive. In addition, it is applicable to many kinds of organic compounds. However, this method cannot be applied in the case of water-soluble organic dyes or barely water-soluble organic compounds, such as pigments. However, Tripathy et al. (1998) recently introduced a new approach for fabricating well-defined organic microcrystals through electrostatic self-assembly. Similarly, Baba et al. (2000) introduced a novel microwave-irradiation process to improve the conventional reprecipitation method.
Most of the current research focuses on the preparation of free-standing organic microcrystals, and far less work has been conducted on inorganic semiconductor microcrystals. These are organic microcrystals with sizes ranging from a few tens to a few hundred nanometers, and they have received substantial attention in the past few decades (Tripathy et al., 1998; Oikawa et al., 2000). This interest arises largely from the observation that their unique size-dependent optical, electronic, and chemical properties differ from those of either their single molecules or their bulk crystals (Jortner and Rao, 2002; Chiang et al., 2012; Indrasti et al., 2020). The excitement of nanoscience and the potential applications of these microcrystals are now driving intensive research activities, and organic nano-/microcrystals are now finding a broad range of applications in the areas of lithography (Magdassi and Moshe, 2003), organic photoconductors (Miyashita et al., 2008) and pharmaceutical formulations for drug delivery and biomedical applications (Kumar and Lal, 2014).
Typically, organic microcrystals have a high surface energy and large curvature, which changes the chemical nature of the organic molecules on the surface and renders the small organic microcrystals thermodynamically unstable. For this reason, obtaining organic nano-/microcrystals even with sizes below 100 nanometers is very difficult, and this size is still much larger than that of semiconductor or noble metal nano-/microcrystals (Li et al., 2017). Therefore, much research has focused on the use of additives, capping agents, or inorganic or polymer matrices to control the size and shape of organic microcrystals (Abyan et al., 2005). One approach has been the use of Schiff bases containing imine group (?CH=N?) as these bases have structural peculiarities and wide pharmacological and biological applications (Fasina et al., 2013; Mondal et al., 2017).
Schiff bases are usually
formed by the condensation of a primary amine with a carbonyl compound (an
aldehyde or ketone) in the presence of an acid catalyst. A careful survey of
the literature shows only limited reports on the fabrication of Schiff-base
micro-scale crystals by reprecipitation methods (Mahajan
et al., 2017; Patil et al., 2017; Kumar et al., 2019). In recent years,
interest has grown in the chemistry of S-alkyl/aryl dithiocarbazates
owing to their potential biological activities and their great behavior as
intermediates in organic reactions (Malik et al.,
2020). To the best of our knowledge, the fabrication of this type of
Schiff base by a reprecipitation method has not been reported previously.
Therefore, this paper summarizes the details of a preliminary investigation on
the fabrication, characterization and antibacterial studies of a S-benzyldithiocarbazate
Schiff base and its microcrystals.
This study provides some new insight into the microcrystallization of Schiff base compounds by the reprecipitation method using an organic solvent-water system. Adjusting the concentration of the target compound in the organic solution and the temperatures of the microcrystallization process was confirmed as a feasible and practical way to control the size and quality of the organic microcrystals. The rate and efficiency of dispersion of the organic solvent in water govern the microcrystal size. These insights into microcrystallization by the reprecipitation method would allow further development of organic microcrystal fabrication in the Rayleigh regime with fully controlled properties. Varying the concentration of TASBnDTC with acetone at 0.001, 0.002, and 0.003 M yielded microcrystals of the compound with the same crystal structure. Interestingly, this preliminary study showed that these Schiff base microcrystals have slight antibacterial activities against four strains of bacteria (inhibition zone ranges of 8.0 to 10.75 mm), whereas the bulk molecular form has no antibacterial activity at all. Therefore, the Schiff base microcrystals warrant further studies by varying the moiety of the Schiff base as well as exploring its minimum inhibition concentration. These studies are currently underway in our laboratories.
We are grateful to the Chemical Sciences [FIC Grant No. UBD/RSCH/1.4/FICBF(b)/2020/024], the Environmental and Life Sciences, Geosciences and Centre for Advanced Material and Energy Sciences, Universiti Brunei Darussalam for the necessary support in carrying out the research work.
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