Cyanide Usage in the Global Industry

Cyanide usage in the global industry is inevitable. The material is extensively used in metallurgy, mining, synthetic fibers, resins, dyes, and electroplating 1. However, cyanide is extremely poisonous at very low doses in humans 2,3. The cyanide anions supplied by the decomposition of toxic cyanide in the human body can quickly combine with the hemoglobin in red blood cells, prevent oxygen transfer, and promote organism death. The World Health Organization recommends 1.9 ?M as the threshold safety level of cyanide in drinking water 3,4. In this regard, cyanide anion detection has attracted scientific concern worldwide.

Over the past decades, many approaches, such as electrochemical, potentiometric, and voltammetry, have been explored to determine cyanide anion levels in aqueous solutions 5–7. However, the use of optical chemosensors in cyanide detection involves a short response time and simple procedures relative to those of other methods 8–10. Thus, research on this field has attracted increased attention given the fast response and highly precise recognition of cyanide ions via this strategy and the evidence-based confirmation of its sensing effectiveness by comprehensive reviews.

Novel cyanide-sensing technologies have been developed. Examples include new cyanide receptors, such as near-infrared chemodosimeters based on 5,10-dihexyl-5,10-dihydrophenazine; 16 dual-mode probes based on coumarin and malonyl urea derivative dyes;13 aggregation-induced emissive hexaphenylbenzene 14 and 7,7,8,8-tetracyanoquinodimethane; 15 4,4-bis-[3-(4-nitrophenyl) thiourea] diphenylmethane (or ether); 12and glucoconjugated o-(carboxamido) aldehyde hydrazine-linked azo dye 11

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The formation of optical chemosensors via confining of highly sensitive and selective organic receptors into mesoporous 17-21 or microporous surfaces is a significant development in practical applications. In general, physical 22-26 and chemical 27 immobilizations are the two major processes in fabricating optical chemosensors. The physical trapping of colorant j-aggregate receptors is uncomplicated. However, unfavorable orientations and diminished functionality are expected in the colorant j-aggregate receptors.

Owing to the discharge of the organic receptors in the solution, the chemosensors grafted via chemical immobilization procedures have short lifep. 28 The binding assays of chemosensors onto porous scaffold is the most efficient approach for constructing chemosensors with long lifetime and highly reproducible response. 29 One disadvantage of chemical immobilization is the unregulated covalent binding of optical receptors to a surface. This drawback may restrict the carrier's active site.

To quantify and track the ultra-trace toxicant species in biological cells, numerous analytical techniques were applied. The monitoring/tracking of toxicants in living cells can be studied on either fixed or live models. These analytical techniques including spatially resolved mass spectrometry techniques 30 such as, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron spectroscopy imaging (ESI) and secondary ion mass spectrometry (SIMS), combined with electron energy loss spectroscopy (EELS), 31,32 and X-ray fluorescence microscopy (XRFM) significantly enabled the distribution and concentration of toxicants in static biological specimens.

33 However, these analytical protocols often associated with practical training, skillfully technical demands in their handling and working may lead to high operating cost of analysis and difficulty for daily routine monitoring of species.

In the last several years, metal–organic frameworks (MOFs) have attracted research attention. 34 Highly crystalline and highly porous MOFs are compounds composed of organic linkers an metal ions to fabricate 1D, 2D, or 3D structures. The organic linker structure arrangement controls the fabrication of the porous structure and augments the specific surface area. 34–37 MOFs possess improved potential compared with other sensors for various applications, such as gas storage/ adsorption; 38 catalysis, 39 platforms for nanomaterials, 40 and luminescence.

41 Furthermore, nanomaterial platforms of inorganic-organic frameworks, which are loaded with optical and fluorescent probes, in designing of chemosensors can be utilized for detecting toxicant in different water sources. 42

In the present work, the branching molecular architectures (BMAs) showed potential applications in colorant tracking and biocompatible cyanide monitoring of ultra-trace concentrations (up to 88 parts per trillion [ppt]) of toxic CN? ions in living cells, i.e., HeLa cells, in the order of seconds. We investigated structural morphology and the stability, of wrapping-colorant buildings and their potential as selective platforms for accommodating dendritic colorant (branch) aggregates (L1).

The thin-layered coating films of colorant aggregates (branch) around and within the molecular building voids of inorganic–organic framework carriers allowed for the hierarchical engineering of BMAs and afforded continuous monitoring of toxic CN? ions in living cells (Scheme 1). For monitoring and visualization of CN? ions in living cells, our results investigated the superior performance of our tailored BMAs. Furthermore, our finding provided a cutoff evidence for the biocompatibility and low cytotoxicity of BMAs during the continuous monitoring and exposure to HeLa cells. Moreover, our result provided a new direction in the application of organic–inorganic framework carriers to environmental and biological sample analyses.

Experimental Section

Fabrication design of branching molecular architectures (BMA)

A simple one-pot solvothermal approach was performed to fabricate supermicropores Al-based metal-organic frameworks using water and N, N-dimethylformamide (DMF) as mixed solvent. The Al(NO3)3. 9H2O (0.51 g), 2-amino terephthalic acid (0.56 g) were dissolved in the mixed solvent and stirred for only 5 minutes. Under static conditions, the reactants were transferred to a Teflon-lined autoclave and heated for 24 h at 160?C. The obtained white powder was filtered and washed with mixed solvent.

To get ride off the excess of organic linker trapped within the micropores, the fabricated Al-MOFs were stimulated in boiling methanol overnight and kept for 24 h at 80?C. The branching molecular architectures (BMA) were constructed via a direct immobilization method process. The 500 mg of Al-MOFs were added to ethanolic solution of the L1 receptor (10 mg) with stirring at room temperature till saturation. At ambient temperature, the solvent is removed gently, leading to change in the carrier color from white to pale yellow which shows that the direct stacking of L1 receptor into the microporous surface of the Al-MOFs.

The decoration process repeated for several times to confirm the filling of microporous surface of Al-MOFs and adsorption capacity reached the equilibrium. The BMA was washed with Milli Q water until no elution of receptor color was detected and dried to 60oC for 12h. The Field emission scanning electron microscopy (FE-SEM), scanning transmission electron microscopy (FESEM), N2 adsorption-desorption isotherms and Small- and wide-angle powder X-ray diffraction (SAXRD, and WAXRD, respectively) (see supporting information) were used to characterize the structural and surface morphology of the microporous (BMA) structures.

Cell culture and in vitro study

The HeLa cells line was gained from Hela (ATCC® CRL1721™) and cultured by incubation at 37 °C under 5% CO2 in Dulbecco's Modified Eagle's Medium (DMEM) containing 10% horse serum and 10% fetal bovine serum (FBS). The culture medium was exchanged every 3-4 days.

Results and discussion

Building blocks of branching molecular architectures (BMAs)

We investigated the fabrication of inorganic–organic aluminum frameworks with a controllable geode-shelled nanorod formation via a one-pot, template-free and simple assays. The hierarchical engineering of BMA geodes was successfully constructed via direct physical immobilization of organic colorant (dendritic branch) aggregates into bowls of swirled caves of geode-shelled nanorod carriers for the continuous monitoring and colorant tracking of CN? ions in real biological and environmental samples.

The multifunctional surfaces of BMAs unveiled uniform swirled caves along the nanorods branches in neatly branched structures. Under solvothermal conditions, the organic linker such as terephthalic acid (BDC), plays a critical role in the fabrication of uniform hollow-nest architecture of high specific surface areas platforms. The fabricated BMAs were designed via physical immobilization. 42–50. At room temperature, the hydrogen bonding and van der Waals interactions occurred between the multifunctional surface of the carrier and colorant aggregates (Scheme 2).

The direct decoration created uniform BMAs without pore blockage with interior and outer surface dressing. The organic nature of the carriers and swirled caves along the branches of the carrier enhanced the binding interactions between the colorant j-aggregate of L1 receptor molecules and the active center of the nanorods. The CN? ion binding affinity of BMAs increased during fast recognition (within seconds) under specific sensing conditions.
Scheme 2

Structural characterization of BMAs

The field-emission scanning electron microscopy (FE-SEM) graphs investigated the formation of the hierarchal geodes of the nest-shaped carriers, which randomly distributed and combined as deep cave-like pockets (Figures 1A–1B). The 50–100 nm nanorods were orders of asymmetrical open-pore systems that connected the cage cavities across the entire geode-shelled nanorod structures.

Figure 1 confirms the formation of cliff swallow nest with massive mouths, chaotically shaped window, and well-arranged nanorods. The scanning transmission electron microscopy images showed an obvious hallow interior structure and the development of uniformly microporous cages to confine the organic colorant aggregate (L1) (Figures 1C–1E).

The poly-nodules of the organic–inorganic frameworks aggregated and were uniformly accumulated around the hollow spheres. The large hollow nest and ridge cavities offered multiple diffusion microchannels for confining organic colorant into spherical geodes, leading to tapping and release of toxic CN? via simple chemical process of the carrier.

The atomic force microscopy image (Figure 1G) showed the stability of the well-ordered organic–inorganic frameworks with hierarchal geode shells. The encapsulation of the organic colorant L1 with no curvature of the shells improved the availability and toxicants ion transportation into the aluminum organic–inorganic frameworks. The external layers formed through the decoration of organic receptors (dendritic branch) around the BMAs. The key components of the building blocks of the BMAs were as follows:

1. The swirled cave structures of the aluminum organic–inorganic framework carriers with randomly shaped channels and active facet {101} enabled the trapping and confinement of organic colorant aggregates.
2. A completely uniform formation of virtual and dense wrapping-colorant architectures.
3. The dressing/wrapping of the organic receptor L1 inside the deep cave-like pockets and nanorod microchannels enabled the fabrication of branching molecular Architectures
4.  The external varnish of the colorant-wrapping aggregates of L1 offered rapid (within seconds), continuous monitoring, and binding of cyanide toxic compounds.

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