Tunable Aminooxy-Functionalized Monolayer-Protected Gold Clusters for Non-Polar or Aqueous Oximation Reactions

Abstract

Aminooxy (-ONH2) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy ‘click chemistry’ on the surface of gold nanoparticles. We present here a trifunctional amine-containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer protected clusters (Au MPCs). Diethanolamine is readily transformed into an organic-soluble aminooxy thiol (AOT) ligand using a short synthetic path. The synthesized AOT ligand was coated on ≤ 2 nm diameter hexanethiolate (C₆S)-capped Au MPCs using a ligand exchange protocol to afford organic-soluble AOT/C₆S (1:1 ratio) Au mixed monolayer protected clusters (MMPCs). This work describes the synthesis of Au(C₆S)(AOT) MMPCs and representative oximation reactions with various types of aldehyde-containing molecules, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.

Graphic Abstract

The work demonstrates ‘click chemistry’ on the surface of Au monolayer-protected clusters (MPCs) using a solubility-switchable trifunctional amine-containing aminooxy alkanethiol. The ligand was incorporated onto <2 nm diameter Au MPCs and subsequently reacted with aldehyde-functionalized molecules of diverse properties. This highlights versatile oximation reactions on Au clusters with potential applications in sensing, separations, and catalysis.

1. Introduction

Aminooxy-based click^[1] chemistry has proven to be a versatile means of ligation, as evidenced by broad application in the fields of material science, biology, biochemistry, analytical chemistry and nanoscience.^[2–4] Aminooxy groups (RONH₂) react chemoselectively with aldehydes and ketones under mild conditions to form highly stable oxime ether adducts. The chemoselectivity of the oximation reaction has stimulated efforts to exploit oxime ether formation not only as a straightforward, efficient coupling approach, but also as a means for identification and quantification of carbonyls in complex mixtures. For example, aminooxy derivatization reagents have been used to selectively tag and/or sequester carbonyl substrates in biological extracts,^[5,6] environmental air and water,^[7,8] exhaled breath,^[9,10] and even living organisms.^[11]

As the usefulness of oximation chemistry has become apparent, several approaches exploiting nanoparticles fitted with aminooxy surfaces have been reported.^[12–14] The ease of functionalization of gold surfaces, clusters, and nanoparticles with a monolayer of thiol-or dithiol-containing molecules^[15,16] has guided some efforts toward developing and applying thiol ligands containing aminooxy functionality.^[17] For example, Nagahori et al. prepared aminooxy-functionalized gold nanoparticles (NPs) for capture and enrichment of glycosphingolipid (GSL)-generated aldehydes as a means to characterize whole GSLs in living cells.^[18] Thygesen et al. developed a bifunctional thiol-aminooxy oligo(ethylene glycol) ligand for preparation of aminooxy-conjugated gold glycanoparticles.^[19] Maynard and coworkers synthesized a photo-caged aminooxy alkane thiol for conjugating carbonyl substrates to gold surfaces following photolysis.^[20] Given the rapidly growing interests in using gold monolayer-protected clusters (Au MPCs) and Au NPs for applications in catalysis,^[21–23] sensing volatile organic compounds,^[24,25] analyzing small molecule mixtures using NP-mediated Raman and laser desorption/ionization spectroscopy,^[26] and drug delivery,^[27,28] the development of aminooxy-functionalized thiol ligands to improve ease of carbonyl ligation as well as to provide flexibility in addressing solubility and adduct loading considerations would enable new applications. With this in mind, we designed a trifunctional amine-containing aminooxy thiol (AOT) ligand for use in mono-layer functionalization of hexanethiolate (C₆S)-protected Au MPCs to produce Au(C₆S)(AOT) mixed monolayer protected clusters (MMPCs) (Figure 1). Incorporation of the amine group was motivated by several considerations: a) the solubility properties of the cluster may be adjusted by control over the amine protonation or alkylation state; b) the amine moiety enables convenient structural bifurcation to increase pendant aminooxy density; and c) an ammonium NH could potentially accelerate oximation reactions.^[29] Herein we describe the synthesis of such a trifunctional thiol ligand and its use in the formation of Au MMPCs. We also present our findings on representative oximation reactions between Au(C₆S)(AOT) MMPCs and various types of aldehyde-containing molecules under both non-polar and aqueous conditions to impart futher functionality into the Au clusters for future catalysis and sensing applications.

Figure 1.

Concept for tuning the solubility properties of Au(C₆S)(AOT) MMPCs.

2. Results and Discussion

2.1. Ligand and Aldehyde Syntheses.

Synthesis of the bis(aminooxy) thiol ligand AOT was accomplished by first N-alkylating diethanolamine (1) by heating with 11-bromo-1-undecene (Scheme 1). Subsequent transformation of the primary alcohol moieties to phthalimido groups was performed using standard Mitsunobu conditions according to the method reported by Grochowski and Jurczak.^[30] Installation of the terminal thiol moiety to form 4 was next attempted, also using established conditions.^[31] AIBN-initiated radical addition of thioacetic acid to the terminal alkene of 3 proved troublesome; however, on protonation of the amine group using CSA prior to addition of AIBN, the reaction produced thioacetate 4 in good yield. Treatment of 4 with excess hydrazine hydrate cleaved both phthalimide groups to reveal the aminooxy moieties as well as cleaved the thioester group to afford AOT.

Scheme 1.

Synthesis of bis(aminooxy) thiol ligand AOT. Conditions: a. i. 11-bromo-1 undecene, CH₃OH:CH₃CN (3:7), reflux, 48 h; ii. aq. NH₄OH, 88 %; b. N-hydroxyphthalimide, PPh₃, diisopropyl azodicarboxylate, toluene, 0 °C to rt, 16 h, 95%; c. i. camphorsulfonic acid, THF, 0 °C, 2 h, ii. azobisisobutyronitrile, thioacetic acid, reflux, 2 0 h, iii. aq. NaHCO₃, 78%; d. hydrazine monohydrate, CH₂Cl₂, 0 °C to rt, 12 h, 82%.

As part of our program on Au MMPCs-based chemiresistors for carbonyl detection, we sought to develop a convenient route for the assembly of ureafunctionalized Au clusters. We previously reported on the effectiveness of a terminal urea functionality for sensing acetone.^[32] Thus, to investigate a new synthetic approach using aminooxy chemistry, we prepared a model urea-aldehyde for coupling with the Au(C₆S)(AOT) MMPCs. Synthesis of urea-aldehyde 9 (Scheme 2) was performed by first masking the alcohol group of 8-amino-1-octanol (6) by reaction with TMSCl to afford silyl ether 7, which was then treated with tert-butyl isocyanate to connect the urea moiety. Silyl ether deprotection using TBAF afforded urea alcohol 8 in good yield. Subsequent oxidation using PCC gave urea-aldehyde 9.

Scheme 2.

Synthesis of urea-aldehyde 9. Conditions: a. Me₃ SiCl, Et₃N, THF, 0 °C to rt, 12h, 85%; b. t-BuNCO, Et₃N, CH₂Cl₂, 0 °C to rt, 18h; c. Bu₄NF, THF, 8 °C to rt, 5h, 69% (2 steps); PCC, NaOAc, SiO₂, CH₂Cl₂, 0 °C to rt, 5h, 63%.

2.2. Ligand Place Exchange Reactions.

A key step in functionalization of Au MPCs with aminooxy groups is the ligand place exchange reaction. All efforts to directly incorporate the AOT ligand during Au MPC formation uniformly failed due to problems with aggregation and cluster precipitation, likely due to the strong interparticle hydrogen bonding that increases as AOT loading increases. Indeed, this challenge is evident from examination of the literature, which indicates that aminooxy-containing ligands are widely incorporated as aminooxy-protected ligands followed by deprotection of the resultant functionalized surfaces to reveal the aminooxy groups.^[18–20] We were gratified to discover that a ligand place exchange reaction smoothly incorporates AOT to deliver aminooxy-functionalized Au MMPCs. First, we used a slightly modified literature procedure to synthesize hexanethiolate (C₆S)-capped Au MPCs.^[33] Thermogravimetric analysis (TGA) on the resultant C6S Au MPCs showed an organic composition of 20.25% (Figure S1, Supporting Information), which is close to the expected 19.84% weight change for a Au144(C₆S)₆₀ cluster, and the absence of a localized surface plasmon resonance band in the UV-vis spectrum is consistent with clusters below 2 nm in diameter (Figure S2, Supporting Information). With C₆S Au MPCs in hand, a ligand place exchange reaction was performed to prepare Au(C₆S)(AOT) MMPCs with a ~1:1 ratio of the reactant thiol (AOT):parent thiol (C₆S) by simple mixing C₆S Au MPCs with AOT (Scheme 3).^[34] Based on ¹H NMR analysis (Figure 2A–C), there is clear evidence of exchange as new, broad peaks with chemical shifts indicative of AOT were observed in the spectrum of the isolated Au(C₆S)(AOT) MMPCs. Specifically, the distinctive ¹H NMR signals at δ 3.76 (-CH₂ONH₂), 2.68 (-N(CH₂CH₂ONH₂)₂) and 2.53–CH₂N(CH₂CH₂ONH₂)₂ ppm in the spectrum of the organic soluble MMPCs confirm the replacement of C₆S with AOT. Integration of the C₆S methyl resonance at 8 0.86–0.89 ppm with the unique downfield AOT signal at δ 3.76 (Figure 2C, blue arrow) indicates incorporation of AOT at a ~1:1 ratio of AOT: C₆S. UV-VIS spectroscopy on the MMPCs showed that the size of the clusters remained less than 2 nm in diameter after the exchange reaction, as no plasmon band was observed (Figure S3, Supporting Information). To demonstrate the availability of the aminooxy groups in these MMPCs for oximation chemistry, we next examined the conjugation of various aldehydes.

Scheme 3.

Schematic representation of place exchange and oximation reactions using the AOT-derived Au MMPCs; L = linker, AOT = aminooxy thiol.

Figure 2.

¹H NMR spectra (400 MHz, CDCl₃) of (A) C₆SAu MPCs, (B) thiol ligand AOT, (C) Au(C₆S)(AOT) MMPCs, and (D) benzaldehyde- and (E) propanal-adducts obtained on reaction with Au(C₆S)(AOT)MMPCs. Peaks at δ 7.24 ppm are residual chloroform (solvent) peaks.

2.3. Oximation Reactions on Au(C₆S)(AOT) MMPCs.

We examined the conjugation of a representative panel of aldehydes that included benzaldehyde (organic-soluble aldehyde), ferrocene carboxaldehyde (redox active aldehyde), pyrene carboxaldehyde (fluorophore), propanal and glyceraldehyde (water-soluble aldehydes), and urea-aldehyde 9 (bifunctional aldehyde). The oximation reactions were performed under mild conditions in either THF or water as solvent to accommodate the specific carbonyl compounds (Scheme 3). The progress of the coupling is readily monitored by ¹H NMR analysis. Specifically, the distinctive downfield shifts of the methylene group adjacent to the aminooxy moiety on oxime ether formation (blue arrows, Figures 2D–E, Figure 3) as well as appearances of characteristic oximyl protons (-CH=NOR, red arrows) for the E- and Z-oxime ether products show that the Au(C₆S)(AOT) MMPCs reacted efficiently with the panel of aldehydes to afford the corresponding oxime ether adducts. Oxime ether formation was also confirmed by FTIR spectroscopy, which showed a loss of the carbonyl C=O stretching frequency near 1700 cm⁻¹ for the starting aldehyde molecule to a C=N stretching frequency in the 1600–1640 cm⁻¹ range for the oxime ether-functionalized MMPCs (Table 1 and corresponding FTIR spectra in Figures S4–S9). The size of the reacted Au(C₆S)(AOT) MMPCs did not change as a result of the oximation reaction as shown by the UV-VIS spectrum after reacting with benzaldehyde (Figure S10, Supporting Information).

Figure 3.

¹H NMR spectra (400 MHz, CD₃OD) of (A) glyceraldehyde- and (B) urea-aldehyde 9-adducts obtained on reaction with Au(C₆S)(AOT) MMPCs. Peaks at δ 4.87 and 3.34 ppm are residual methanol (solvent) peaks.

Table 1.

IR stretching frequencies confirming the coupling of various aldehydes to Au(C₆S)(AOT) MMPCs through an oximation reaction.

R-CHO	Starting aldehyde C=O (cm−1)	oxime ether MMPCs C=N (cm−1)
Ph-CHO	1696	1604
CH3CH2-CHO	1734	1633
CH2(OH)CH(OH)-CHO	1731	1599
CpFe(C5H4)-CHO	1658	1607
pyrene-CHO	1678	1602
urea-aldehyde 9	1723	1634

To demonstrate oximation under aqueous conditions, Au(C₆S)(AOT) MMPCs were reacted with propanal and glyceraldehyde in pH 4.0 acetate buffer solutions. The clusters readily dissolved, likely due to protonation of the amino moiety rendering the Au MMPCs water-soluble (Scheme 3), and reacted efficiently with the water-soluble aldehydes to yield the corresponding oxime ether adducts as mixtures of E/Z isomers (Figures 2E and 3A). In the case of propanal, neutralization of the adduct solution by washing with basic buffer restored the organic solubility and enabled extraction of the derivatized MMPCs into organic solvent for ease of isolation.

The functionalization of Au(C₆S)(AOT) MMPCs with redox molecules and fluorophores was assessed by cyclic voltammetry (CV) and fluorescent measurements, respectively. For example, after reaction of Au(C₆S)(AOT) MMPCs with ferrocene (Fc)-carboxaldehyde (Figure 4A), electrochemical measurements on the Fc-derivatized MMPCs in solution (Figure 4B, red CV) demonstrated oxidation/reduction behaviour characteristic of ferrocene. The small 45 mV peak splitting and symmetric waves indicate that the Au(C₆S)(AOT-ferrocene) MMPCs are strongly adsorbed to the Pt working electrode surface, similar to previous results on other Fc-MPCs with high loading.^[35,36] A CV of the working electrode after removal from the solution of Au(C₆S)(AOT-ferrocene) MMPCs and placing in solvent and electrolyte only (Figure 4B, blue CV) confirms strong adsorption of these MMPCs to the electrode surface since the Fc peaks remain in the CV and they are symmetric with small peak splitting. The fluorophore 1-pyrenecarboxaldehyde was attached to the Au(C₆S)(AOT) MMPCs by simple mixing and the fluoresence of the resulting MMPCs was measured. Comparison of the fluorescence emission spectra of 1-pyrenecarboxaldehyde with the corresponding derivatized Au(C₆S)(AOT-pyrene) MMPCs with the same pyrene concentration shows the fluorescence is quenched when bound to the Au cluster (Figure 5). The quenching of fluorophores by metal nanoparticles is well established,^[37] and serves in this case as an indication of successful coupling of fluorophores through oximation.

Figure 4.

(A) Structure of ferrocenecarboxaldehyde adduct obtained on reaction with Au(C₆S)(AOT) MMPCs and (B) cyclic voltammograms of Au(C₆S)(AOT-ferrocene) MMPCs in a 0.10 M TBAHFP/CH₂Cl₂ solution with an overall ferrocene concentration of 1.7 mM (red) and of the electrode after having been removed from the Au MMPCs solution and then placed in a 0.10 M TBAHFP/CH₂Cl₂ electrolyte solution (blue). Scan rate = 100 mV/s, working electrode = Pt disk (2 mm diameter), counter electrode = Pt wire, and reference electrode = Ag wire quasi-reference electrode.

Figure 5.

Fluorescence emission spectra of 1-pyrenecarboxaldehyde (1.0 E-07 M) (blue), Au(C₆S)(AOT-pyrene) MMPCs (1.0 E-07 M in pyrene-CHO) (pink), and CHCl₃ (red). Excitation λ = 375 nm.

3. Conclusions

This work describes significant loading (50%) of Au MPCs with a new bifurcated aminooxy-thiol ligand featuring a tertiary amino group for solubility tuning. The aminooxy thiol ligand was directly loaded onto Au MPCs using a ligand place exchange protocol that obviates the need for aminooxy protection. Simple mixing of the derived aminooxy Au MMPCs in either organic or aqueous media with a panel of aldehydes demonstrated the ease of incorporation of a variety of functionalities onto the cluster surface. Multiple methods of characterization, including ¹H NMR, IR, CV, fluorescence and UV-vis spectroscopy, confirmed that oxime ether bond formation proceeded efficiently and that particle size was not affected by the oximation reactions. This work importantly lays the groundwork for our future plans to utilize the Au(C₆S)(AOT) MMPCs as both a capturing agent and electrical transducer for chemiresistive and optical sensing of aldehyde- and ketone-containing volatile organic compounds of environmental and biomedical importance. We also aim to functionalize the Au(C₆S)(AOT) Au MMPCs with water-soluble poly-carbonyls to controllably synthesize caged metal catalysts, where the metal is the catalyst and the polymer coating acts as a filter to control selectivity. This simple method will be benefical to other groups for general cluster/NP functionalization or applications in sensing, separations, or catalysis.

4. Experimental Section

4.1. Materials and methods.

All chemicals were purchased either from Sigma-Aldrich (Saint Louis, MO), Bean Town Chemical (Hudson, NH) or Alfa Aesar (Tewksbury,MA) unless otherwise noted. All solvents were freshly distilled before use. HAuC1₄·3H₂O was synthesized from metallic bulk gold according to the literature procedure.^[38] Thin layer chromatography was performed using silica plates (silica gel 60G F254). UV active compounds were visualized by UV light (254 nm). The TLC plates were stained using either iodine or p-anisaldehyde. Water was purified using a Barnstead water ultra-purification system (ThermoFisher, resistivity of 18.2 MQ·cm) and used whenever required.

4.2. Spectroscopic measurements.

¹H and ¹³C NMR spectra were recorded either on a Varian 400 MHz or Varian Inova 500 MHz spectrometer. Infrared spectra were recorded on a Perkin-Elmer Spectrum FT-IR spectrophotometer with an attenuated total reflectance attachment. UV-Vis spectra were recorded on a Varian instrument model CARY 50 Bio UV-Visible spectrophotometer. All scans were obtained between 300 nm to 900 nm wavelength range at a fast scan rate in a glass cuvette and the background was subtracted using CHCl₃ or nanopore H₂O as the blank. High resolution mass spectrometry (HRMS) of new compounds was performed using a Thermo Scientific Q Exactive Focus Orbitrap LC-MS/MS system. The ¹H NMR, ¹³C NMR, and FTIR for all compounds and Au MPCs or MMPCs not discussed directly in the main text are provided in Supporting Information in Figures S11–S33.

4.3. Thermogravimetric analysis (TGA).

Material compositions of the synthesized clusters were determined by measuring the weight changes using a thermogravimetric analyzer (TA Instruments, TA 2050) under a nitrogen atmosphere from 25 to 800 °C with a heating rate of 5 °C min⁻¹. This provides an approximate cluster molecular formula when combined with the known cluster diameter.

4.4. Electrochemical measurements.

The redox potential of ferrocene (Fc)-attached clusters was measured by cyclic voltammetry (CV) using a CH Instruments CHI 660A electrochemical workstation (Austin, TX) with the following electrode configurations: working electrode – 2 mm diameter Pt disk; counter electrode – Pt wire; and reference electrode – Ag wire (QRE). The supporting electrolyte solution was prepared using tetrabutylammonium hexafluorophosphate (TBAHFP). The Fc-coupled Au cluster (10 mg) was added to a 0.1 M solution of TBAHFP (0.39 g, 1.00 mmol) in CH₂Cl₂ (10 mL) to yield a solution for CV measurements (1.70 mM total Fc concentration).

4.5. Fluorescence Measurements.

Fluorescence measurements were performed using a Perkin Elmer LS55 fluorescence spectrometer with CHCl₃ as a blank in a quartz cuvette. For the study with 1-pyrenecarboxaldehyde (1-PyCHO), intensity measurements were performed at [1-PyCHO] = 1.0 × 10⁻⁷M with an excitation wavelength of 375 nm. The Au(C₆S)(AOT–pyrene) MMPCs (2.0 mg) were added to CHCl₃ (20 mL). 8.5 μL of this solution then was added to 10 mL CHCl₃ to yield a solution for fluorescence measurements (1.0 × 10⁻⁷M total pyrene concentration).

4.6. Synthesis procedures for aminooxy-thiol ligand.

Scheme 1 outlines the synthesis of the aminooxy-thiol ligand (AOT).

4.6.1. 2-[(2-Hydroxyethyl)-N-10-undecenylamino] ethanol (2).

A solution of diethanolamine (1) (0.5 mL, 5.20 mmol) and 11-bromo-1-undecene (1.25 mL, 5.72 mmol) in a 30:70 mixture of methanol:acetonitrile (20 mL) was heated 48h at reflux. The reaction mixture was cooled to rt and the solvents were removed by rotary evaporation. The residue was triturated with diethyl ether (3 × 20 mL) and the solids dried under high vacuum to obtain the intermediate ammonium bromide (1.02 g, 3.00 mmol) as a yellow viscous oil, which then was dissolved in water (20 mL) and followed by addition of 2N ammonium hydroxide solution (20 mL). The solution was transferred to a separatory funnel and was extracted with Et₂O (3 × 20 mL). The combined ether extract was concentrated by rotary evaporation and the residue placed under high vacuum overnight to afford diol 2 (1.18 g, 88 %) as a waxy solid; IR (neat) 3351, 2924, 2853, 1640, 1464 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 5.75–5.83 (m, 1H), 4.90–5.00 (m, 2H), 2.49–2.53 (t, J = 12.0 Hz, 2H), 2.63–2.66 (t, J = 4.0 Hz, 4H), 3.59–3.62 (t, J = 8.0 Hz, 4H), 2.02–2.05 (m, 2H), 1.26–1.43(m, 14H) ppm; ¹³C NMR (100 MHz, CDCl₃,) δ 139.1, 114.0, 59.5, 56.0, 54.8, 33.7, 29.5, 29.3, 29.0, 28.8, 27.3, 26.9 ppm; HRMS m/z [M + H]⁺ calcd for C₁₅H₃₂NO₂⁺ 258.2428, found 258.2419.

4.6.2. 2-(2-{[2-(3-Oxo-2-isoindolinoyloxy)ethyl]-N-10-undecenylamino}ethoxy)-1,3-isoindolin-dione (3).

To a solution of N-hydroxyphthalimide (0.456 g, 2.80 mmol) and triphenylphosphine (0.734 g, 2.80 mmol) in toluene (40 mL) at 0 °C was added dropwise a solution of diol 2 (0.30 g, 1.17 mmol) dissolved in toluene (5 mL). After stirring 1 hour at 0 °C, diisopropyl azodicarboxylate (0.551 mL, 2.8 mmol) was added slowly via syringe. The reaction mixture was stirred an additional 1 hour at 0 °C and followed by stirring 16h at rt. The solvent then was removed by rotary evaporation and the residue was dissolved in ethyl acetate (20 mL) and the resultant solution was washed with saturated aq. NaHCO₃ (5 × 20 mL), water (20 mL) and brine (2 × 20 mL). The organic layer was concentrated, and the crude material was then cooled to 0 °C. Cold 5% aqueous HCl (5 mL) was added followed by stirring the mixture 45 minutes at rt. The aqueous slurry was extracted with diethyl ether (5 × 20 mL) and then cooled to 0 °C before adjusting the pH to 7–8 by slow addition of saturated aq. NaHCO₃. The alkaline aqueous layer was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic extract was dried (anhydrous Na₂SO₄), filtered, and concentrated by rotary evaporation to afford 3 (0.60 g, 95%) as a light yellow solid that was used in the next step without further purification; mp 86–88 °C; IR (solid) 2921, 2850, 1789, 1733, 1466, 1378 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.73–7.80 (m, 8H), 5.77–5.85 (m, 1H), 4.91–5.00 (m, 2H), 4.31–4.33 (t, J = 4.0 Hz, 4H), 3.06–3.08 (t, J = 4.0 Hz, 4H), 2.63–2.67 (t, J =8.0 Hz, 2H), 2.02–2.05 (t, J = 6.0 Hz, 2H), 1.62 (m, 2H) 1.25–1.42 (m, 12H) ppm; ¹³C NMR (100 MHz, CDCl₃) δ 163.2, 139.1, 134.3, 128.9, 123.4, 113.9, 76.6, 54.8, 52.0, 33.7, 29.6, 29.3, 29.0, 28.8, 27.1 ppm; HRMS m/z [M + H]⁺ calcd for C₃₁H₃₈N₃O₆⁺ 548.2755, found 548.2740.

4.6.3. S-(11-(Bis(2-((1,3-dioxoisoindolin-2-yl)oxy)ethyl)amino)undecyl)ethanethioate (4).

To a solution of bis(phthaloyloxy)amine 3 (0.465 g, 0.849 mmol) in THF (10 mL) at 0 °C was added camphorsulfonic acid (0.296 g, 1.27 mmol) to form the corresponding ammonium salt and stirred at 0 °C for 2h before allowing it to warm to rt for 30 minutes while degassing. The reaction mixture was heated at 40 °C for 15 minutes before slow addition of a solution of AIBN (182 mg, 0.594 mmol) dissolved in dry THF (2 mL) via syringe and followed by addition of thioacetic acid (131 μL, 1.86 mmol) via syringe. The resultant mixture was degassed for 20 min using a stream of argon before heating the solution at reflux for 20h. The reaction mixture was then cooled to rt, quenched with diethyl ether (10 mL) and concentrated by rotary evaporation to afford the crude product, which was dissolved in CHCl₃ (20 mL). The CHCl₃ solution was washed three times with saturated aq. NaHCO₃ (20 mL), followed by washing with brine (20 mL), and the organic layer was dried with anhydrous sodium sulfate. The solvents were removed by rotary evaporation to afford a light-yellow solid residue that then was triturated with hexane (3 × 20 mL) to give 4 as a pale yellow solid (0.41 g, 78%); mp 96–98 °C; IR (solid) 2919, 2849, 2795, 1788, 1736, 1690, 1466 cm⁻¹; ¹H NMR (500 MHz, CDCl₃,) δ 7.70–7.80 (m, 8H), 4.30–4.32 (t, J= 6.0 Hz, 4H), 3.05–3.07 (t, J= 5.5 Hz, 4H), 2.83–2.86 (t, J = 7.5 Hz, 2H), 2.62–2.65 (t, J = 7.5 Hz, 2H), 2.30 (s, 3H), 1.52–1.55 (qui, J = 7.0 Hz, 2H), 1.23–1.41 (m, 16H) ppm; ¹³C NMR (125 MHz, CDCl₃) 5 195.9, 163.2, 134.2, 129.2, 123.3, 76.6, 54.7, 52.2, 30.5, 29.4, 29.0, 28.7, 27.5 ppm; HRMS m/z [M + H]⁺ calcd for C ₃₃H₄₂N₃O₇S⁺, 624.2738, found 624.2731.

4.6.4. 11-(Bis(2-(aminooxy)ethyl)amino)undecane-1-thiol (AOT).

To thioester 4 (0.546 g, 0.876 mmol) in CH₂Cl₂ (10 mL) at 0 °C was added slowly hydrazine monohydrate (0.260 mL, 5.25 mmol) via syringe. The resultant reaction mixture was stirred at 0 °C for 30 min. before stirring at rt for 12 h. The precipitated phthalyl hydrazide was removed by filtration and the filtrate was concentrated by rotary evaporation to afford crude compound 5. The residue was purified with silica gel column chromatography using CH₂Cl₂/methanol (9.5:0.5) to give the bis(aminooxy) thiol AOT as a viscous oil (0.230 g, 82%); IR (neat) 3306, 2922, 2852, 1590, 1464 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 5.30 (m, 4H), 3.70–3.73 (t, J = 4.0 Hz, 4H), 2.67–2.70 (t, J = 4.0 Hz, 4H), 2.61–2.64 (t, J = 4.O Hz, 2H), 2.44–2.48 (m, 3H), 1.53–1.61 (m, 2H), 1.41 (m, 2H), 1.21–1.32 (m, 14H) ppm; ¹³C NMR (100 MHz, CDCl₃) δ 73.4, 55.1, 52.9, 39.0, 33.9, 29.4, 29.1, 28.9, 28.4, 27.3, 26.7, 24.5 ppm; HRMS m/z [M + H]⁺ calcd for C₁₅H₃₅N₃O₂S 322.2530, found 322.2518; [M + Na]⁺ calcd for C₁₅H₃₅N₃NaO₂S⁺ 344.2347, found 344.2336.

4.7. Synthesis procedures for urea-aldehyde 9.

Urea-aldehyde 9, a substrate for a Au MMPCs-based chemiresistor, was prepared as outlined in Scheme 2.

4.7.1. 8-((Trimethylsilyl)oxy)octan-1-amine (7).

To a solution of 8-amino-1-octanol (6) (0.94 g, 6.47 mmol) in THF (30 mL) at 0 °C was added successively trimethylsilyl chloride (0.985 mL, 7.76 mmol) and triethylamine (Et₃N) (1.53 mL, 10.9 mmol) via syringe while under argon. The reaction mixture was stirred at 0 °C for 2h and then allowed to warm to rt. After 12h, the reaction was diluted with diethyl ether (30 mL) and the precipitated solids were filtered. The filtrate was concentrated, dissolved in ethyl acetate (50 mL), successively washed with sat’d aq. NaHCO₃ (3 × 50 mL) and brine (2 × 50 mL), and then dried (Na₂ SO₄). The solvents were removed by rotary evaporation to afford amine 7 (1.18 g, 85%) as a pale yellow waxy solid suitable for use without further purification; mp 60–62 °C; IR (solid) 3393, 3356, 2922, 2851, 1561, 1445, cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 3.55 (s, br, 2H), 2.69 (s, br, 2H), 1.70 (s, br, 2H), 1.44–1.51 (m, br, 4H), 1.30 (m, 8H), 0.10 (s, 9H) ppm; ¹³C NMR (100 MHz, CDCl₃) δ 62.8, 48.9, 42.1, 33.6, 32.7, 29.6, 29.3, 26.7, 25.6 ppm; HRMS m/z [M + H]⁺ calcd for C₁₁H₂₈NOSi⁺ 218.1935, found 218.1932.

4.7.2. 1-(tert-Butyl)-3-(8-hydroxyoctyl)urea (8).

To amine 7 (0.955 g, 4.39 mmol) in dry CH₂Cl₂ (20 mL) under argon at 0 °C was added slowly triethylamine (672 μL, 4.83 mmol) and then tert-butyl isocyanate (508 μL, 4.39 mmol) via syringe. The resultant reaction mixture was stirred at 0 °C for 30 min before stirring at rt for 18h. The crude solution so obtained was concentrated by rotary evaporation and used directly in the next step without further purification.

To a stirred solution of crude urea intermediate dissolved in dry THF (25 mL) under argon at 0 °C was added tetrabutylammonium fluoride (5.6 mL of 1.0 M solution in THF, 5.27 mmol) via syringe. The resultant reaction mixture was stirred at 0 °C for 30 min before stirring at rt for 5h. The crude mixture was diluted with deionized water (25 mL), extracted with CH₂Cl₂ (2 × 20 mL) and the combined organic layer then was dried (Na₂ SO₄) before concentrating via rotary evaporation. The residue was purified by SiO₂ flash column chromatography eluting with a 3:1 mixture of CH₂Cl₂ : EtOAc (R_f = 0.42) to give urea-alcohol 8 as a white solid (0.740 g, 69%); mp 78–80 °C; IR (solid) 3361, 3230, 2967, 2923, 2851, 1652, 1563, 1478, 1463 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 3.63 (t, J = 6.0 Hz, 2H), 3.09 (t, J = 7.2 Hz, 2H), 1.53–1.57 (m, 2H), 1.40–1.47 (m, 4H), 1.30–1.32 (m, 17H); ¹³C NMR (100 MHz, CDCl₃) δ (ppm), 157.9, 63.3, 50.7, 40.7, 33.1, 30.6, 29.9, 29.6, 29.5, 27.1, 25.9; HRMS m/z [M + H]⁺ calcd for C₁₃H₂₉N₂O₂⁺ 245.2224, found 245.2218; [M + Na]⁺ calcd for C₁₃H₂₈N₂NaO₂+ 267.2043, found 267.2039.

4.7.3. 1-(tert-Butyl)-3-(8-oxooctyl)urea (9).

A suspension of alcohol 8 (0.720 g, 2.94 mmol), pyridinium chlorochromate (PCC) (2.22 g, 10.3 mmol), silica gel (2.90 g), and sodium acetate (0.846 g, 10.3 mmol) in dry THF (20 mL) at 0 °C was stirred for 1h before warming to rt. After 4h, the reaction mixture was diluted with diethyl ether (30 mL) and the resultant solution was filtered through a short column of celite. The filtrate was dried (Na₂ SO₄) and the solvents then removed by rotary evaporation. The residue was purified by SiO₂ flash column chromatography eluting with 3:1 mixture of CH₂Cl₂ : EtOAc (R_f = 0.52) to afford urea-aldehyde 9 (0.450 g, 63%) as a pale yellow viscous liquid; IR (neat) 3362, 2928, 2857, 1723, 1632, 1555, 1451cm⁻¹; ¹H NMR (500 MHz, CDCl₃) δ 9.73 (s, 1H), 4.60–4.66 (NH, 2H), 3.07 (m, 2H), 2.40 (m, 2H), 1.60 (m, 2H), 1.42 (m, 2H), 1.23–1.35 (m, 17H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 203.2, 158.1, 50.3, 44.0, 40.3, 30.4, 29.8, 29.2, 26.9, 22.1 ppm; HRMS m/z [M + H]⁺ calcd for C₁₃H₂₇N₂O₂⁺ 243.2067, found 243.2065.

4.8. Synthesis of C₆SAu MPCs.

Hexanethiolate-functionalized monolayer-protected gold clusters (C₆SAu MPCs) with a ~1.6 nm diameter were prepared using a 3:1 molar ratio of 1-hexanethiol: HAuCl₄·3H₂O. The synthesis was carried out following the conditions reported by Brust et al. with slight modifications.^[33] In a typical reaction, tetraoctylammonium bromide (0.305 g, 0.559 mmol) was dissolved in toluene (60 mL) and stirred for 15 min. before addition of a solution of HAuCl₄·3H₂O (0.200 g, 0.507 mmol) in water (20 mL). The biphasic mixture was stirred vigorously for 30 minutes to ensure full transfer of HAuCl₄·3H₂O from the water to the toluene phase. The mixture then was transferred to a separatory funnel and the aqueous layer was removed. The organic layer was transferred to a clean flask and 1-hexanethiol (216 μL, 1.52 mmol) was added followed by stirring 45 minutes at rt, whereupon a color change from orange to colorless was observed. The reaction mixture was cooled to 0 °C using an ice bath and a solution of sodium borohydride (0.192 g, 5.07 mmol) in water (20 mL) was rapidly added to form a black solution, indicative of cluster formation. The reaction mixture was further stirred at 0 °C for 30 minutes and then stirred at rt for 4h, whereupon it was transferred to a separatory funnel and the aqueous layer was removed. The organic layer was concentrated to near dryness by rotary evaporation and acetonitrile (ca. 40 mL) was added. The solid black MPCs (0.140 g) were collected after 12h by vacuum filtration using a fritted glass funnel after washing with acetonitrile and finally drying in air (0.140 g). The clusters were characterized by UV-VIS, TGA, FTIR and ¹H NMR spectroscopy, which was consistent with pure C₆S-protected Au MPCs with an approximate composition of Au₁₄₄(C₆S)₆₀ as described in the literature.^[39]

4.9. Place exchange reactions to prepare MMPCs.

We prepared gold mixed monolayer-protected clusters (MMPCs) functionalized with mixtures of hexane-thiolate and AOT by performing place exchange reactions on the Au₁₄₄(C₆S)₆₀ MPCs. In a typical procedure, the AOT ligand (96.7 mg, 0.301 mmol) and the Au₁₄₄(C₆S)₆₀ MPCs (80.1 mg, 0.150 mmol) were mixed under argon in dry THF and stirred at rt for 3d (Scheme 3). The resultant, crude Au(C₆S)(AOT) MMPCs were concentrated by rotary evaporation, suspended in acetonitrile (10 mL), and then purified by repeated centrifugation and supernatant removal (x3). Characterization by UV-VIS, IR (solid), and ¹H NMR revealed that the composition of the Au MMPCs was approximately Au₁₄₄(C₆S)₃₀(AOT)₃₀.

4.10. Oximation Reactions.

Both non-polar and aqueous oximation reactions were performed on Au₁₄₄(C₆S)₃₀(AOT)₃₀ MMPCs with a panel of aldehydes. The overall reaction design and conditions are depicted in Scheme 3.

4.10.1. Benzaldehyde adduct.

To a solution of Au₁₄₄(C₆S)₃₀(AOT)₃₀MMPCs (25.6 mg, 0.0185 mmol of AOT) dissolved in dry THF (2.0 mL) was added benzaldehyde (7.55 μL, 0.0740 mmol) followed by stirring at rt for 12h. The reaction mixture was concentrated by rotary evaporation and the residue was suspended in acetonitrile (10 mL). The oxime ether-modified MMPCs then were isolated by centrifugal precipitation. The process of washing using acetonitrile followed by centrifugation was repeated two times. The isolated Au(C₆S)(AOT-benzaldehyde) MMPCs (20.5 mg) were characterized by IR and ¹H NMR spectroscopy (Figure 2).

4.10.2. Ferrocenecarboxaldehyde adduct.

Ferrocenecarboxaldehyde (31 mg, 0.14 mmol) was added to the Au(C₆S)(AOT) MMPCs (50.5 mg, 0.0365 mmol of AOT) dissolved in dry THF (2.5 mL) at rt with stirring. After 12h, the reaction mixture was concentrated by rotary evaporation and the residue was suspended in acetonitrile (10 mL). The oxime ether adduct was isolated by centrifugal precipitation. The process of washing using acetonitrile followed by centrifugation was repeated two times. The isolated Au(C₆S)(AOT-ferrocene) MMPCs (43.6 mg) were characterized by IR and ¹H NMR spectroscopy. The redox behavior of the cluster was studied by cyclic voltammetry measurements (Figure 4).

4.10.3. 1-Pyrenecarboxaldehyde adduct.

1-Pyrenecarboxaldehyde (40 mg, 0.17 mmol) was added to the Au(C₆S)(AOT) MMPCs (60.5 mg, 0.0437 mmol of AOT) dissolved in dry THF (3.0 mL) at rt while stirring. The reaction mixture was purged with argon and stirred at rt for 18h. Upon completion, the reaction mixture was concentrated by rotary evaporation and the residue was suspended in acetonitrile (10 mL). The oxime ether adduct was isolated by centrifugal precipitation. The process of washing using acetonitrile followed by centrifugation was repeated two times. The isolated Au(C₆S)(AOT-pyrene) MMPCs (50.5 mg) were dried under high vacuum overnight and characterized by IR, ¹H NMR, and fluorescence spectroscopy (Figure 5).

4.10.4. Propanal adduct.

To a stirred solution of the Au(C₆S)(AOT) MMPCs (20.5 mg, 0.0148 mmol AOT) in pH 4.0 acetate buffer solution (5.0 mL) at rt was added propanal (6.5 μL, 0.0888 mmol). The mixture was stirred at rt for 18h before addition of pH 8.0 phosphate buffer (10 mL). The resultant suspension was extracted using THF (3 × 10 mL). The combined organic extract was concentrated by rotary evaporation and the residue was dried under high vacuum overnight. The Au(C₆S)(AOT-propanal) MMPCs (16.9 mg) were characterized by IR and ¹H NMR spectroscopy (Figure 2).

4.10.5. Glyceraldehyde adduct.

To a stirred solution of the Au(C₆S)(AOT) Au MMPCs (45.0 mg, 0.0325 mmol AOT) in pH 4.0 acetate buffer solution (10 mL) was added DL-glyceraldehyde (29.3 mg, 0.325 mmol). After stirring at rt for 1d, a pH 8.0 phosphate buffer solution (20 mL) was added. The resultant suspension was freeze dried and then lyophilized. The obtained residue was suspended in acetonitrile (10 mL) for 1d followed by removal of the supernatant. Centrifugal precipitation using successive acetonitrile washings (3 × 10 mL) followed by drying under high vacuum overnight afforded Au(C₆S)(AOT-glyceraldehyde) MMPCs (35.9 mg). The MMPCs were characterized by IR and ¹H NMR spectroscopy (Figure 3).

4.10.6. Urea-aldehyde 9 adduct.

To a solution of the Au(C₆S)(AOT) MMPCs (76.8 mg, 0.0555 mmol of AOT) dissolved in dry THF (3.0 mL) was added urea-aldehyde 9 (53.8 mg, 0.222 mmol) followed by stirring rt for 22h. The reaction mixture was concentrated by rotary evaporation and the residue was suspended in acetonitrile (10 mL) for 1 day. The oxime ether-modified MMPCs then were washed by repeated centrifugal precipitation and supernatant removal steps using acetonitrile (3 × 10 mL). The isolated Au(C₆S)(AOT-ureaaldehyde) MMPCs (70.2 mg) were characterized by IR and ¹H NMR spectroscopy (Figure 3).