Diazonium Gold Salts as Novel Surface Modifiers: What Have We Learned So Far?

The challenges of diazonium salts stabilization have been overcome by their isolation as metal salts such as tetrachloroaurate(III). The cleavage of molecular nitrogen from diazonium salts even at very low potential or on reducing surfaces by fine tuning the substituents on the phenyl ring expanded their applications as surface modifiers in forensic science, nanomedicine engineering, catalysis and energy. The robustness of the metal–carbon bonding produced from diazonium salts reduction has already opened an era for further applications. The integration of experimental and calculations in this field catalyzed its speedy progress. This review provides a narrative of the progress in this chemistry with stress on our recent contribution, identifies potential applications, and highlights the needs in this emerging field. For these reasons, we hope that this review paper serves as motivation for others to enter this developing field of surface modification originating from diazonium salts.


Introduction
Diazonium salts are outstanding surface modifiers at low electrochemical reduction potentials, chemical-less or even spontaneously [1][2][3]. The easily and rapidly formed salts particularly in the last few years could produce a wide variety of diazonium salts modified with functional groups such as amine, carboxyl, maleimide, boronic acid, thiol, and many others [2]. In 1992, Pinson and co-workers started one of the pioneering works to understand the mechanism of aryl radical attachment to glassy carbon electrodes [1]. The aryl-substrate interface with a true covalent bonding was a crucial step to use aryldiazonium salts as surface coupling agents and modifiers in material sciences for adhesive resins, biomacromolecules, polymers, optical sensing purposes, and nanoparticles for catalysis [2]. Scheme 1 shows aryldiazonium salts and their applications in several fields.
Preparation of aryldiazonium salts can be conducted from an aromatic amine in excess mineral acid media and addition of an aqueous solution of sodium nitrite which needs to be kept at a temperature below 5 • C. Aryldiazonium salts have many serious drawbacks in synthesis and stability: difficulty to purify and isolate, explosive nature, and substantial instability at room temperature [2,3]. To avoid these limitations, usually they are used in situ to the target application without isolation. For aliphatic diazonium salts, limited success in the synthesis of stable compounds has been reported because they are prone to release molecular nitrogen under experimental conditions [4].
The two major routes to modify the nanoparticles are either using isolated or in situ generated aryldiazonium salts. Because of their ability to modify nanoparticles spontaneously, isolated aryldiazonium salts are highly interesting compounds to study. However, the long-term stability as solids and solutions has been a major concern. In situ generated aryldiazonium salts, instead of using isolated ones, can achieve high grafting densities of aryl layers with high colloidal stability. The robustness of aryl-capped gold nanoparticles can be at least for six months fabricated from in situ synthesized diazonium salts [5]. The ability to isolate pure and stable aryldiazonium salts plays an important factor in nanoparticles-aryldiazonium interface progress. However, various stable diazonium salts have been isolated and their X-ray structures have been reported [6,7]. For example, arenediazonium tosylates were prepared using a polymer-supported diazotization agent with p-toluenesulfonic acid. These salts showed outstanding stability that could be stored for long terms to use for subsequent transformation reactions [7].

Metal Nanoparticles Modification with Aryls from Aryldiazonium Salts
Metal nanoparticles modified with aryls from aryldiazonium salts can be prepared by one of these methods: (1) a spontaneous reduction of aryldiazonium salts on nanoparticles surface, (2) phase-transfer of aryldiazonium salts to metal salts, (3) reduction of aryldiazonium salts in the presence of metal salts, and (4) reduction of aryldiazonium salts bearing metal-based anions. We demonstrated in our group, for the first time, that we can synthesize stable diazonium tetrachloroaurate(III) salts [4-R-C 6 H 4 N 2 ] + AuCl 4 − (R = CN, C 8 F 17 , NO 2 , F, Cl, Br, I, C 6 H 13 , water-soluble dendrons) in a high yield [8][9][10]. For example, protonation of 4-CN-C 6  produced ruby red gold-carbon nanoparticles [9]. Functionalization of metal nanoparticles can be achieved by the attachment of organic shells with the desired functionality on their surfaces, which can affect the physical and chemical properties of the nanoparticles. Aryldiazonium modification of flat surfaces and nanoparticles occurs through direct metal-carbon, metal-oxygen-carbon, or metal-diazenido-carbon bonding. In addition, grafting depends on the nature of the metal surface and pre-treatment conditions [2]. Aryl shell can be adsorbed on metal oxide nanoparticles surfaces. However, in the case of MnO 2 nanorods modification Downard concluded from XPS studies on the in situ formed film from diazonium salts in acetonitrile solution or using isolated diazonium salt 4-nitrobenzenediazonium salt in 0.1 M NaOH that covalent Mn-O-C bonding, through surface oxygen, to MnO 2 nanorods is formed. The possible formation of Mn-C bonds cannot be discounted [11].
Both the aryldiazonium salts chemistry and the photopolymerization iniferter methods were utilized in the synthesis of aluminum-carbon nanoparticles [12]. The functionalization of Al nanoparticles was performed by a bifunctional initiator, 4-[(diethylcarbamothioyl)thio] methylbenzenediazonium salt (DEDTC-CH 2 -C 6 H 4 -N 2 + ), containing a diazonium end group for surface anchoring and a N,N-diethyldithiocarbamate (DEDTC) to activate surface-initiated photoiniferter-mediated polymerization (SIPIMP). Surface analysis shows that a thin aluminum oxide layer was preserved on the Al core and a strongly anchored organic shell surrounds it. XPS analysis of the modified Al nanoparticles showed the polymerization thickness over time reached 3-4 nm after 6 hrs. After functionalization, the oxide layer thickness decreased 57% and the C/Al atomic ratio increased progressively from 1.1 to 13.6, indicating an efficient coating by the polymer. Figure 1 testifies a clear enhancement of the metallic Al core contribution at~72 eV relative to Al 2 O 3 at~75 eV in the presence of the aryl coating. Apparently, the diazonium approach has shown some restrictions due to the limited solubility of the diazonium salts in nonpolar solvents, preferred over polar protonated solvents, due to their inertness toward aluminum nanoparticles obtained from aluminum hydride.
Surfaces 2020, 3 FOR PEER REVIEW 3 Both the aryldiazonium salts chemistry and the photopolymerization iniferter methods were utilized in the synthesis of aluminum-carbon nanoparticles [12]. The functionalization of Al nanoparticles was performed by a bifunctional initiator, 4-[(diethylcarbamothioyl)thio] methylbenzenediazonium salt (DEDTC-CH2-C6H4-N2 + ), containing a diazonium end group for surface anchoring and a N,N-diethyldithiocarbamate (DEDTC) to activate surface-initiated photoiniferter-mediated polymerization (SIPIMP). Surface analysis shows that a thin aluminum oxide layer was preserved on the Al core and a strongly anchored organic shell surrounds it. XPS analysis of the modified Al nanoparticles showed the polymerization thickness over time reached 3-4 nm after 6 hrs. After functionalization, the oxide layer thickness decreased 57% and the C/Al atomic ratio increased progressively from 1.1 to 13.6, indicating an efficient coating by the polymer. Figure  1 testifies a clear enhancement of the metallic Al core contribution at ~72 eV relative to Al2O3 at ~75 eV in the presence of the aryl coating. Apparently, the diazonium approach has shown some restrictions due to the limited solubility of the diazonium salts in nonpolar solvents, preferred over polar protonated solvents, due to their inertness toward aluminum nanoparticles obtained from aluminum hydride. Trials by Nishihara and collaborators to utilize diazonium salts in vanadium modification produced diazenido (R-N=N-V) modified vanadium nanoparticles [13]. The construction of the nanoparticles was carried out by the reduction of VCl4 with LiBHEt3 in the presence of 4octylphenyldiazonium salt. XPS measurements provided clear evidence of the V-N bonding at approximately 396.5 eV. Recently, iron nanoparticles of 21 nm size were synthesized at room temperature by sodium borohydride reduction of iron(III) chloride in the presence of 4nitrobenzenediazonium salt in an aqueous medium [14]. The synthesized nanoparticles maintained air stability and high oxidation resistance for at least 6 months.

Aryldiazonium Gold Salts for Flat and Nanoparticulate Surface Modification
Electrochemical reductive grafting of diazonium salts has been the privileged method for the modification of surfaces using aryl functional groups [15][16][17][18][19][20][21][22][23][24]. The reduction potential for the salts is very low, which aids in the modification process of a given surface without bringing further complications. Gold(III) diazonium salts showed a lower reduction potential on a glassy carbon electrode comparing with diazonium tetrafluoroborate [8]. The electrochemical reductive grafting works of the aryldiazonium gold(III) salts are in parallel with ongoing studies in the preparation of robust gold-organic nanoparticles, and the results provides rich foundation for covalent functionalization of gold surfaces [24]. Cyclic voltammetry (CV) studies of 1.0 mM of diazonium gold (III) salts of [4-X-C6H4N2] + AuCl4 − , (X = F, Cl, Br, I, CN, NO2 and C8F17), and bis(diazonium) gold(III) salts of syn-and anti-geometry of 4,4'-(1,3-phenylenediisopropylidene)bisaniline and of 4,4'-(1,4 phenylenediisopropylidene)bisaniline in 0.1 M tetrabutylammonium hexafluorophosphateacetonitrile (TBAHFP-CH3CN) showed highly irreversible reduction peaks at 100 mV/s scan rate in the range of 0.00 to -0.22 V vs. Ag/AgCl, KCl (satd.), both on the glassy carbon (GC) and platinum Pt working electrodes [21][22][23][24]. For all salts, multi cyclic voltammetry studies of ten consecutive cycles Trials by Nishihara and collaborators to utilize diazonium salts in vanadium modification produced diazenido (R-N=N-V) modified vanadium nanoparticles [13]. The construction of the nanoparticles was carried out by the reduction of VCl 4 with LiBHEt 3 in the presence of 4-octylphenyldiazonium salt. XPS measurements provided clear evidence of the V-N bonding at approximately 396.5 eV. Recently, iron nanoparticles of 21 nm size were synthesized at room temperature by sodium borohydride reduction of iron(III) chloride in the presence of 4-nitrobenzenediazonium salt in an aqueous medium [14]. The synthesized nanoparticles maintained air stability and high oxidation resistance for at least 6 months.

Aryldiazonium Gold Salts for Flat and Nanoparticulate Surface Modification
Electrochemical reductive grafting of diazonium salts has been the privileged method for the modification of surfaces using aryl functional groups [15][16][17][18][19][20][21][22][23][24]. The reduction potential for the salts is very low, which aids in the modification process of a given surface without bringing further complications. Gold(III) diazonium salts showed a lower reduction potential on a glassy carbon electrode comparing with diazonium tetrafluoroborate [8]. The electrochemical reductive grafting works of the aryldiazonium gold(III) salts are in parallel with ongoing studies in the preparation of robust gold-organic nanoparticles, and the results provides rich foundation for covalent functionalization of gold surfaces [24]. 0.00 to −0.22 V vs. Ag/AgCl, KCl (satd.), both on the glassy carbon (GC) and platinum Pt working electrodes [21][22][23][24]. For all salts, multi cyclic voltammetry studies of ten consecutive cycles at both the GC and Pt working electrodes exhibited a decrease in the reduction current with an increase in the cycle number, and at the tenth cycle there is almost no reduction current validating that grafting and blocking of the gold-aryl film is complete. A typical case of the Br salt at GC electrode is shown in Figure 2.
Surfaces 2020, 3 FOR PEER REVIEW 4 at both the GC and Pt working electrodes exhibited a decrease in the reduction current with an increase in the cycle number, and at the tenth cycle there is almost no reduction current validating that grafting and blocking of the gold-aryl film is complete. A typical case of the Br salt at GC electrode is shown in Figure 2. In order to determine the blocking efficiencies of the grafted films, CV studies of the reversible redox couple [Fe(CN) 6 ] 3−/4− were conducted using GC and Pt working electrodes prior and after grafting using 10 consecutive cycles in the potential range from 0.80 to −0.80 V vs. Ag/AgCl, KCl (satd.). For all [4-X-C6H4N2] + AuCl4 − salts, the redox peaks of [Fe(CN)6] 3−/4− were completely attenuated due to the complete inhibition of its redox electron transfer by the grafted films on the GC working electrode. For the grafted films on the Pt electrode, the redox peaks of [Fe(CN)6] 3−/4− were not disappearing, illustrating that the blocking and efficiency of the grafted film on Pt electrode is not as good as the GC electrode. Similar studies to check the robustness of the grafted films of all [4-X-C6H4N2] + AuCl4 − salts after one hour sonication in water, acetonitrile, toluene, water, and petroleum ether exhibited the grafted film on the GC electrode is robust. For the Pt electrode, except for petroleum ether, the grafted films were not found as robust as the GC films.
Similar [Fe(CN)6] 3−/4− electron transfer and sonication experiments on the bis(diazonium) gold(III) salts of syn-and anti-geometry of 4,4'-(1,3-phenylenediisopropylidene)bisaniline and of 4,4'-(1,4-phenylenediisopropylidene)bisaniline using GC electrode showed that the grafted film for the syn diazonium salt is efficient and robust. The grafted film of the anti-salt on the GC electrode, however, was not found to be as efficient and robust as the syn salt. The grafted films on the Pt electrode for both the syn and anti bis(diazonium) gold(III) salts were found not to be efficient and robust.
Surface morphology studies conclude that the perfluorinated organic film C8F17-4-C6H4 was grafted on the in situ reduced gold(III) salt [4-C8F17-C6H4N2] + AuCl4 − [23,24]. ATR-FTIR shows the νC-F stretching frequencies of the perfluorinated film and the absence of the νN≡N stretching frequency for the diazonium peak. XPS study of the grafted film displays C1s peaks at 291-294 eV (CF2/CF3). The F1s and the C1s peaks testify for the successful electro-grafting of the fluorous group. Recent CV studies of [4-HOOC-C6H4N2] + AuCl4 − in 0.10 M KNO3 aqueous solution showed a remarkably lower irreversible reduction potential of 0.638 V vs. Ag/AgCl, KCl (satd.) than the other gold(III) diazonium salts, supporting its applicability in the synthesis of nanoparticles via green approach. salts after one hour sonication in water, acetonitrile, toluene, water, and petroleum ether exhibited the grafted film on the GC electrode is robust. For the Pt electrode, except for petroleum ether, the grafted films were not found as robust as the GC films. Similar [Fe(CN) 6 ] 3−/4− electron transfer and sonication experiments on the bis(diazonium) gold(III) salts of syn-and anti-geometry of 4,4 -(1,3-phenylenediisopropylidene)bisaniline and of 4,4 -(1,4-phenylenediisopropylidene)bisaniline using GC electrode showed that the grafted film for the syn diazonium salt is efficient and robust. The grafted film of the anti-salt on the GC electrode, however, was not found to be as efficient and robust as the syn salt. The grafted films on the Pt electrode for both the syn and anti bis(diazonium) gold(III) salts were found not to be efficient and robust.
Surface morphology studies conclude that the perfluorinated organic film C 8 F 17 -4-C 6 H 4 was grafted on the in situ reduced gold(III) salt [4-C 8 F 17 -C 6  irreversible reduction potential of 0.638 V vs. Ag/AgCl, KCl (satd.) than the other gold(III) diazonium salts, supporting its applicability in the synthesis of nanoparticles via green approach. Femtosecond laser ablation of the aryldiazonium gold(III) salt was used in ultrapure gold-aryl nanoparticles construction in acetonitrile [25]. Also, the reduction of triazine dendrons as diazonium gold salts produced robust gold nanoparticles which are soluble in water and toluene [10]. Recently, the synthesis of aryldiazonium tetrachloroaurate(III) salts [4-X-C 6 H 4 N 2 ] + AuCl 4 − (X = F, Cl, Br, I, CN, NO 2 ) and their mild reduction to covalently functionalized nanoparticles was reported [26]. The gold-aryl nanoparticles were also constructed by the mild chemical reduction of the diazonium gold(III) salts. The study showed the presence of the gold(0) in the core and the organic shell connectivity in solid-state and in a solution using various spectroscopic techniques (see Figures 3 and 4). Furthermore, the experimental with theoretical results have been compared from Raman spectroscopy, which both supports the presence of gold-carbon bonding based on DFT calculations.
Surfaces 2020, 3 FOR PEER REVIEW 5 Femtosecond laser ablation of the aryldiazonium gold(III) salt was used in ultrapure gold-aryl nanoparticles construction in acetonitrile [25]. Also, the reduction of triazine dendrons as diazonium gold salts produced robust gold nanoparticles which are soluble in water and toluene [10]. Recently, the synthesis of aryldiazonium tetrachloroaurate(III) salts [4-X-C6H4N2] + AuCl4 − (X = F, Cl, Br, I, CN, NO2) and their mild reduction to covalently functionalized nanoparticles was reported [26]. The gold-aryl nanoparticles were also constructed by the mild chemical reduction of the diazonium gold(III) salts. The study showed the presence of the gold(0) in the core and the organic shell connectivity in solid-state and in a solution using various spectroscopic techniques (see Figures 3, 4). Furthermore, the experimental with theoretical results have been compared from Raman spectroscopy, which both supports the presence of gold-carbon bonding based on DFT calculations.   Reprinted with permission from [26]. © 2020 Elsevier B.V. All rights reserved.
Surfaces 2020, 3 FOR PEER REVIEW 5 Femtosecond laser ablation of the aryldiazonium gold(III) salt was used in ultrapure gold-aryl nanoparticles construction in acetonitrile [25]. Also, the reduction of triazine dendrons as diazonium gold salts produced robust gold nanoparticles which are soluble in water and toluene [10]. Recently, the synthesis of aryldiazonium tetrachloroaurate(III) salts [4-X-C6H4N2] + AuCl4 − (X = F, Cl, Br, I, CN, NO2) and their mild reduction to covalently functionalized nanoparticles was reported [26]. The gold-aryl nanoparticles were also constructed by the mild chemical reduction of the diazonium gold(III) salts. The study showed the presence of the gold(0) in the core and the organic shell connectivity in solid-state and in a solution using various spectroscopic techniques (see Figures 3, 4). Furthermore, the experimental with theoretical results have been compared from Raman spectroscopy, which both supports the presence of gold-carbon bonding based on DFT calculations.
Diazonium salts can easily modify metal surfaces with organic film through electrochemical and electroless grafting methods [35]. Hence, using diazonium salts seems promising to use in crime scenes where it reacts preferentially on the reducing metal surface to generate fingerprints picture. Recently, aryldiazonium gold(III) salt was evaluated in forensic chemistry for latent fingerprint development on different metal surfaces. [4-NO 2 -C 6 H 4 N 2 ] + AuCl 4 − salt was applied with different concentrations to copper, lead, and aluminum surfaces using the components of human sweat as the reducing agent to construct gold deposits on the eccrine fingerprints [36]. The adsorbed gold-aryl film on the metal surfaces showed well-separated ridges of the latent fingerprints in the SEM measurements ( Figure 5). Also, the presence of deposited gold helps to trace the fingerprint on the metal surfaces by XRF and EDS. Visualization of the developed fingerprints was studied by examining the primary, secondary, and tertiary details. Primary details can be obtained fully from the latent fingerprint on copper and lead surfaces. The secondary level is more important which gives more details for recognition like bifurcation, eye shape, island and ridge ends. The images show high quality on copper and lead surfaces due to the good contrast and clear furrows. Figure 6 shows the tertiary details as well. Overall, aryldiazonium gold(III) salt solutions produced a high-quality visualization of the latent fingerprints. The diazonium salts approach for metal surfaces modification allows the depositing gold metal that can be traced by XRF and EDS instruments compared to the electrochromism approach of polyaniline which depends on electrochemical handling [37].
Recently, we used XPS to investigate diazonium salts as surface modifiers of latent fingerprints collected on metal surfaces [38]. In this study, cleaned copper coins and copper sheets were modified by fingerprints of a donor. The fingerprints were developed with gold aryldiazonium ([4-NO 2 -C 6 H 4 N 2 ] + AuCl 4 − ) aqueous solution for 1 h. After washing, the copper coins and flat surfaces were analyzed by XPS. After only 15-20 min, the reaction of the diazonium aqueous solution with copper formed positive dark lines, and bubbles were visible between the fingerprint lines. Figure 7 shows XPS spectra collected from four different areas of the now-visible fingerprint samples. It was easy to differentiate the fingerprint side and the non-fingerprint side of the copper sample by looking at the survey scan and observing the shape of the background in the range of 700-900 eV. Only two peaks for nitrogen were observed, one centered at 400 eV and another at 406 eV assigned to -N≡N-within the aryl layer and NO 2 , respectively. Therefore, the components due to N1s from the diazonium group N 2 + cannot be longer observed due to the spontaneous reduction of the diazonium cation on copper.
Au 4d 5/2 accounts for the metallic state of gold and thus a reductive process affecting [AuCl 4 ] − anions. This combination of XPS and diazonium chemistry to study the development of latent fingerprints without any applied potential or heat offers fast application of fingerprint detection and development and can be further applied to other metal surfaces that have reducing properties.
dissolved in 5% acetonitrile: 95% ethanol solvent after contact for 2 h with (a) Cu surface showing incipient ridges, (b) Pb surface showing smooth edges and (c) Al surface showing some scar structures. Images were taken using stereomicroscope with a 3 MP camera and magnification of 25×-50×. Reprinted with permission from [36]. © 2020 Elsevier B.V. All rights reserved.
Recently, we used XPS to investigate diazonium salts as surface modifiers of latent fingerprints collected on metal surfaces [38]. In this study, cleaned copper coins and copper sheets were modified by fingerprints of a donor. The fingerprints were developed with gold aryldiazonium ([4-NO2-C6H4N2] + AuCl4 − ) aqueous solution for 1 h. After washing, the copper coins and flat surfaces were analyzed by XPS. After only 15-20 min, the reaction of the diazonium aqueous solution with copper formed positive dark lines, and bubbles were visible between the fingerprint lines. Figure 7 shows XPS spectra collected from four different areas of the now-visible fingerprint samples. It was easy to differentiate the fingerprint side and the non-fingerprint side of the copper sample by looking at the survey scan and observing the shape of the background in the range of 700−900 eV. Only two peaks for nitrogen were observed, one centered at 400 eV and another at 406 eV assigned to -N≡N-within the aryl layer and NO2, respectively. Therefore, the components due to N1s from the diazonium group N2 + cannot be longer observed due to the spontaneous reduction of the diazonium cation on copper. Au 4d5/2 accounts for the metallic state of gold and thus a reductive process affecting [AuCl4] − anions. This combination of XPS and diazonium chemistry to study the development of latent fingerprints without any applied potential or heat offers fast application of fingerprint detection and development and can be further applied to other metal surfaces that have reducing properties.

Catalysis
Water-soluble nanoparticles are important for several applications in medicine, catalysis, energy and environment [39][40][41][42][43]. The harsh conditions of these applications like the presence of salts, changing in pH values, and temperature variation requires robust nanoparticles that can maintain their composition. Aryldiazonium salts have been used to modify metal and nanoparticles surfaces [44]. The bonding between the aryl groups and the metal surface is very robust and can withstand harsh conditions such as ultrasonication and boiling in organic solvents [45]. The aryl film formed on the gold surface is very robust and opened a vast area for catalysis applications [46].
The modification of gold nanoparticles with an organic stabilizing robust shell using different aryldiazonium salts grafting method was previously reported [9]. For catalysis, water-soluble gold-aryl nanoparticles were prepared by the reduction of the aryldiazonium gold(III) salt, [4-HOOC-C 6 H 4 N 2 ] + AuCl 4 − . The reduction of 4-nitrophenol (4-NPh) by sodium borohydride (NaBH 4 ) [47] was used to study the reactivity performance of gold-aryl nanoparticles (AuNPs-COOH) as a catalyst. Stability measurements showed high robustness of AuNPs-COOH under different catalytic conditions [48]. Overall, nanoparticles demonstrated high aggregation resistance in a wide pH range of 1-13. The ionic strength effect also studied using NaCl solutions (0.01-1.00 M) where the interaction of the nanoparticles with the ions in NaCl does not aggregate the nanoparticles completely. Thermal stability of functionalized nanoparticles is critical in catalytic applications. AuNPs-COOH stock solution was monitored by UV-Vis in the temperature range 20-90 • C. The nanoparticles also showed high stability with no redshift of the plasmon band in all studied temperature values. DFT calculations estimate the gold-aryl binding energy in Au 38 -C 6 H 4 -COOH model with Cs Symmetry. The results agree well with the robust character with Au-C of 2.04 Å, which is related to the binding energy of −59.2 Kcal/mol. Comparing this result with the binding energy of thiol-Au 38 model, Au-C shows a stronger and more stable bond than the Au-S bond. AuNPs-COOH used as a catalyst in the reduction of 4-NPh by NaBH 4 . The reduction reaction was monitored by UV-Vis spectrophotometer where the nitrophenolate ion has a strong band at 400 nm. Using an excess concentration of NaBH 4 , the reaction follows pseudo-first order kinetics. Figure 8 shows a rapid decrease in the intensity of the absorption peak at 400 nm, while there is an appearance of a new peak at 300 nm assigned to 4-aminophenol (4-APh). The reduction was complete in less than five minutes of 80 µM 4-NPh with a high k app (2.26 × 10 −2 s −1 ). Activation parameters for the reduction reaction was also calculated. The AuNPs-COOH catalyst gives good values of E a = 25 kJ mol −1 , H # = 23 kJ mol −1 and S # = −166 J mol −1 K −1 .

Nanomedicine Engineering
Because of their facile surface functionalization, metal nanoparticles are dominating research studies in the biomedical field. Gold nanoparticles provide a plethora of prospects in theranostics due to their relatively biocompatible nature [49]. By using carboxylic-aryl shell modified gold nanoparticles, insulin could adsorb onto surface of the carboxylic groups [50]. The nanoparticles were synthesized by mixing the aryldiazonium gold(III) salt with insulin in the presence of mild reducing

Nanomedicine Engineering
Because of their facile surface functionalization, metal nanoparticles are dominating research studies in the biomedical field. Gold nanoparticles provide a plethora of prospects in theranostics due to their relatively biocompatible nature [49]. By using carboxylic-aryl shell modified gold nanoparticles, insulin could adsorb onto surface of the carboxylic groups [50]. The nanoparticles were synthesized by mixing the aryldiazonium gold(III) salt with insulin in the presence of mild reducing agents. Figure 9 shows the effect of pH gradient on color, zeta potential, plasmon peak position. The change in wavelength under reversible acidic-neutral-basic values was verified. From the graphs, even after two cycles of pH changes, acidic-neutral-basic, the reversibility is still obvious.

Nanomedicine Engineering
Because of their facile surface functionalization, metal nanoparticles are dominating research studies in the biomedical field. Gold nanoparticles provide a plethora of prospects in theranostics due to their relatively biocompatible nature [49]. By using carboxylic-aryl shell modified gold nanoparticles, insulin could adsorb onto surface of the carboxylic groups [50]. The nanoparticles were synthesized by mixing the aryldiazonium gold(III) salt with insulin in the presence of mild reducing agents. Figure 9 shows the effect of pH gradient on color, zeta potential, plasmon peak position. The change in wavelength under reversible acidic-neutral-basic values was verified. From the graphs, even after two cycles of pH changes, acidic-neutral-basic, the reversibility is still obvious. (D) robustness study of the insulin bioconjugate under reversible change of pH of 2, 7 and 11. Change in color is represented as circles matching the visual color change upon reversible pH shift of two cycles between 2, 7 and 11. Reprinted with permission from [50]. © 2020 Elsevier B.V. All rights reserved.
Another gold bioconjugate was synthesized using bovine serum albumin (BSA) [51]. Because of the similar properties, molecular weight and amino acid sequence to human serum albumin, BSA has been widely used as model protein in bioconjugate fabrication. Using light and phase contrast microscopy, the morphology changes in blood cells and the percentage of hemolyzed cells were studied. The gold-aryl-COOH nanoparticles and Au-BSA bioconjugate showed a superior hemocompatibility probably because of the carboxyl groups.
In the presence of polyelectrolyte polymer, the interaction of aryl-gold nanoparticles with DNA was studied recently [52]. Gold-aryl nanoparticles were treated with biodegradable polydiallyldimethylammonium chloride (PDADMAC) as cationic polyelectrolytes. Nuclease degradation was reduced by DNA binding in the described bioconjugate, which increased its presence in the digestive environment of DNase-I enzyme. Binding and protection against nuclease degradation of DNA were studied using electrophoretic mobility of DNA in agarose gel. Figure 10 shows the bands of gold nanoparticles-DNA bioconjugate were the same as those of the native DNA, showing no change in the electrophoretic mobility of DNA in the presence of unmodified gold nanoparticles. Immobilization of DNA was observed in the presence of PDADMAC-AuNPs, reflecting the efficient complexation of DNA by the cationic PDADMAC coated aryl-gold nanoparticles. The presence of a polymer PDADMAC masks the negative charge of the DNA and provide protection from enzymatic degradation. was studied recently [52]. Gold-aryl nanoparticles were treated with biodegradable polydiallyldimethylammonium chloride (PDADMAC) as cationic polyelectrolytes. Nuclease degradation was reduced by DNA binding in the described bioconjugate, which increased its presence in the digestive environment of DNase-I enzyme. Binding and protection against nuclease degradation of DNA were studied using electrophoretic mobility of DNA in agarose gel. Figure 10 shows the bands of gold nanoparticles-DNA bioconjugate were the same as those of the native DNA, showing no change in the electrophoretic mobility of DNA in the presence of unmodified gold nanoparticles. Immobilization of DNA was observed in the presence of PDADMAC-AuNPs, reflecting the efficient complexation of DNA by the cationic PDADMAC coated aryl-gold nanoparticles. The presence of a polymer PDADMAC masks the negative charge of the DNA and provide protection from enzymatic degradation.

Energy Sector
Diazonium gold salts as bentonite clay surface modifiers in the energy domain for the electrocatalytic hydrogen production in hydrogen evolution reaction (HER) was described [42]. Mirkhalaf used nanostructured electrocatalysts based on diazonium salts immobilized on electrode surfaces and organic film templates for enhancing electrochemical reactions [53]. Diazonium gold salts with perfluorinated chain were used to modify clay surface by the chemical reduction to form clay-gold nanocomposite, Figure 11. The electrocatalytic properties of gold-aryl-clay nanohybrid as efficient and robust catalyst contributes to the ever-growing sustainable energy sector. Addition of gold enhanced the electrochemical hydrogen evolution with a noticeable high current density of −25 mA cm −2 at −1 V (vs Ag/AgCl).
Mirkhalaf used nanostructured electrocatalysts based on diazonium salts immobilized on electrode surfaces and organic film templates for enhancing electrochemical reactions [53]. Diazonium gold salts with perfluorinated chain were used to modify clay surface by the chemical reduction to form clay-gold nanocomposite, Figure 11. The electrocatalytic properties of gold-aryl-clay nanohybrid as efficient and robust catalyst contributes to the ever-growing sustainable energy sector. Addition of gold enhanced the electrochemical hydrogen evolution with a noticeable high current density of −25 mA cm −2 at −1 V (vs Ag/AgCl).  [42]. © 2020 Elsevier B.V. All rights reserved.

Conclusions
Diazonium salts stabilized by metal-based anions are easily reducible. We utilized tetrachloroaurate salt to stabilize diazonium salts under ambient conditions. Gold(III) as a salt has added a few advantages such as the facile reducibility and the formation of gold-carbon of outstanding robustness. Moreover, the functionalization of the nanoparticles surface with different organics facilitated their applications in forensic science for latent fingerprints detection, nanomedicine engineering as protein carriers, catalysts in environmental pollutants removal and recently energy sector. ruby (e) after reduction of gold nanoparticles on bentonite surface as well as the color change of the purified bentonite BP powder from greenish (c) to ruby (f) for both Bp-MPS-ArC 8 F 17 @Au and Bp-DPA-ArC 8 F 17 @Au prepared monohybrids. Reprinted with permission from [42]. © 2020 Elsevier B.V. All rights reserved.

Conclusions
Diazonium salts stabilized by metal-based anions are easily reducible.
We utilized tetrachloroaurate salt to stabilize diazonium salts under ambient conditions. Gold(III) as a salt has added a few advantages such as the facile reducibility and the formation of gold-carbon of outstanding robustness. Moreover, the functionalization of the nanoparticles surface with different organics facilitated their applications in forensic science for latent fingerprints detection, nanomedicine engineering as protein carriers, catalysts in environmental pollutants removal and recently energy sector.