*Article* **Neodymium Selenide Nanoparticles: Greener Synthesis and Structural Characterization**

**Abu A. Ansary <sup>1</sup> , Asad Syed 2,\*, Abdallah M. Elgorban <sup>2</sup> , Ali H. Bahkali <sup>2</sup> , Rajender S. Varma <sup>3</sup> and Mohd Sajid Khan 4,\***


**Abstract:** This investigation presents the greener biomimetic fabrication of neodymium selenide nanoparticles (Nd2Se<sup>3</sup> NPs) deploying nitrate-dependent reductase as a reducing (or redox) agent, extracted from the fungus, *Fusarium oxysporum*. The Nd2Se<sup>3</sup> NPs, with an average size of 18 ± 1 nm, were fabricated with the assistance of a synthetic peptide comprising an amino acid sequence (Glu-Cys)n-Gly, which functioned as a capping molecule. Further, the NPs were characterized using multiple techniques such as UV-Vis spectroscopy, fluorescence, dynamic light scattering (DLS), and XRD. The hydrodynamic radii of biogenic polydispersed Nd2Se<sup>3</sup> NPs were found to be 57 nm with PDI value of 0.440 under DLS. The as-made Nd2Se3NPs were water-dispersible owing to the existence of hydrophilic moieties (-NH<sup>2</sup> , -COOH, -OH) in the capping peptide. Additionally, these functionalities render the emulsion highly stable (zeta potential −9.47 mV) with no visible sign of agglomeration which bodes well for their excellent future prospects in labeling and bioimaging endeavors.

**Keywords:** biomimetic synthesis; Nd2Se<sup>3</sup> ; synthetic peptide; structural characterization

#### **1. Introduction**

Chalcogenides (compounds and alloys of sulfur, selenium, and tellurium) are known for their splendid physical properties as they have phenomenal magnetic, electronic, catalytic, sensing, thermal, optical, and superconductivity properties. The conversions of chalcogenides into nanocrystalline metal chalcogenides have revolutionized the importance of these materials [1]. In particular, semiconductor nanocrystals have attracted global consideration owing to their distinctive optical features, such as great resistance to photo bleaching, an enormous absorption cross-section, extended fluorescence lifetimes, good quantum yield, and a luminescence emission with a large Stokes shift [2]. Furthermore, their perceived properties make them a subject of study for biologically relevant applications, which extend to medicine and bioimaging [3]. Highly functional nanostructured materials are grown using a biological system (including silica via diatoms, and magnetic nanomaterials using magnetotactic bacteria) [4], among numerous ongoing efforts to assemble inorganic nanoparticles (NPs) by biological means thus mimicking such phenomena at a laboratory scale. Various organisms (such as bacteria, fungi, and actinomycetes) have been studied for the generation of NPs over a range of chemical compositions that include metals, semiconductors, and oxides [5–9].

Generally, semiconductor materials are defined by their composition-dependent band gap energy (Eg), which is the minimal energy needed to stimulate an electron from the valence band (VB) into the vacant conduction energy (CE) band [10]. Band gaps play

**Citation:** Ansary, A.A.; Syed, A.; Elgorban, A.M.; Bahkali, A.H.; Varma, R.S.; Khan, M.S. Neodymium Selenide Nanoparticles: Greener Synthesis and Structural Characterization. *Biomimetics* **2022**, *7*, 150. https://doi.org/10.3390/ biomimetics7040150

Academic Editors: Stanislav N. Gorb, Giuseppe Carbone, Thomas Speck and Andreas Taubert

Received: 11 September 2022 Accepted: 29 September 2022 Published: 3 October 2022

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substantial role in determining the purity of nanomaterials as well as in designing the fabrication of devices (sensors and photocatalysts) [11]. Other vital factors that may also affect the development of nanostructures in semiconductor NPs, include the comparative surface energies of the constituent metals, their corresponding rates of surface diffusion, and the conditions prevailing during or after deposition, whereas the large surface area enhances the surface properties. Additionally, these factors make whole-structure characterization important for these systems; moreover, they provide a more effective way to tune the structural morphology, and its corresponding properties. The relative differences between the energetic and fundamental structural parameters of a constituent's metal and nonmetal vary considerably between elemental systems. For the last two decades, the applications of nano-chalcogenides have garnered worldwide attention and their nanocrystalline forms have received immense significance in various applications [12]. Therefore, different routes for the synthesis of nanocrystalline metal chalcogenides have evolved for the preparation of nano-chalcogenides of desired physical properties [13–15]. Metal chalcogenides can easily be tuned, and their optoelectronic, physicochemical, magnetic, and biological properties can conversely manipulate for energy to biomedical applications. Neodymium oxides nanomaterials are being extensively used in optical, antireflection coatings, gate insulators, protective coatings photonic, catalytic, and many special applications [9]. Inner transition metals nanomaterials have attractive luminescent properties. The promising utilities in time resolved luminescence bioassays of lanthanide compounds propose exceptional sharp fluorescence with highly distinguishable long lasting emission bands and large Stoke shifts [16]. Their luminescence is created by transition within 4f shell. To the best of our knowledge, this is the first ever fungal-derived protein-mediated biosynthesis of Nd2Se<sup>3</sup> NPs which may help open new dimensions of fluorescence applications in nanomedicine.

Chemical and physical methods produce highly toxic, unsafe, environmental unfriendly, cumbersome, unstable colloidal NPs that also require tedious derivatization protocols. In this regard, it is highly necessary to develop high-quality semiconductor nanomaterials via cost-effective, convenient, and less toxic methods. Therefore, nanotechnologists are looking toward greener machineries to produce nontoxic, stable, and long-lasting nanomaterials that function under ambient conditions. To that end, various methodologies have been designed to produce biogenic nanomaterials. In addition to this, the green synthesis of Au, Ag, and CdS NPs have also been achieved using enzymes and peptides purified from fungus [17–19]. These synthesized NPs (green synthesis by enzyme) are capped with small metal binding peptides with flanking free carboxyl or amino groups [19]. These readily available functionalities obviate the need for biofunctionalization, which is required for conjugating the NPs to the biomolecules [19]. Particularly, a wide range of templates has been engaged for enzyme immobilization, namely silica nanotubes [20], phospholipid bilayers [21], and self-assembled monolayers [22]. Furthermore, numerous studies have been reported on semiconductor–enzyme conjugates that embrace the development and enzyme-mimicking behavior of NPs complexed with horseradish peroxidase [23], xanthine oxidase [24], and carbonic anhydrase [25].

Herein, our study presents a detailed investigation into the use of enzymes (e.g., nitrate reductase) cleansed from fungus (e.g., *Fusarium oxysporum*) for the fabrication of technically vital, highly stable semiconductor Nd2Se<sup>3</sup> NPs capped with a synthetic peptide. We have observed that the aqueous Nd2+ and Se4+ ions were simultaneously oxidized and reduced (redox reaction), respectively with the aid of enzyme nitrate reductase, leading to the creation of stable Nd2Se<sup>3</sup> NPs in solution; ensued NPs were in the range of 16–27 nm in size, with an average of 18 ± 1 nm. This is an unprecedented use of an enzyme nitrate reductase and capping peptide in the synthesis of Nd2Se<sup>3</sup> NPs. Lastly, these investigations will help assist in developing a rational enzymatic approach for the fabrication of NPs with a variety of compositions as well as tunable surface morphologies.

### **2. Materials and Methods**

All the chemical reagents were acquired from commercially available sources and were of the highest purity available. The enzyme nitrate reductase was purified as described by Kumar et al. [18].
