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Summary

A facile protocol is presented to functionalize the surfaces of nanodiamonds with polydopamine.

Abstract

Surface functionalization of nanodiamonds (NDs) is still challenging due to the diversity of functional groups on the ND surfaces. Here, we demonstrate a simple protocol for the multifunctional surface modification of NDs by using mussel-inspired polydopamine (PDA) coating. In addition, the functional layer of PDA on NDs could serve as a reducing agent to synthesize and stabilize metal nanoparticles. Dopamine (DA) can self-polymerize and spontaneously form PDA layers on ND surfaces if the NDs and dopamine are simply mixed together. The thickness of a PDA layer is controlled by varying the concentration of DA. A typical result shows that a thickness of ~5 to ~15 nm of the PDA layer can be reached by adding 50 to 100 µg/mL of DA to 100 nm ND suspensions. Furthermore, the PDA-NDs are used as a substrate to reduce metal ions, such as Ag[(NH₃)₂]⁺, to silver nanoparticles (AgNPs). The sizes of the AgNPs rely on the initial concentrations of Ag[(NH₃)₂]⁺. Along with an increase in the concentration of Ag[(NH₃)₂]⁺, the number of NPs increases, as well as the diameters of the NPs. In summary, this study not only presents a facile method for modifying the surfaces of NDs with PDA, but also demonstrates the enhanced functionality of NDs by anchoring various species of interest (such as AgNPs) for advanced applications.

Introduction

Nanodiamonds (NDs), a novel carbon-based material, have attracted considerable attention in recent years for use in various applications^1,2. For instance, the high surface areas of NDs provide excellent catalyst support for metal nanoparticles (NPs) because of their super-chemical stability and thermal conductivity³. Furthermore, NDs play significant roles in bio-imaging, bio-sensing, and drug delivery due to their outstanding biocompatibility and nontoxicity^4,5.

To efficiently extend their capabilities, it is valuable to conjugate functional species on the surfaces of NDs, such as proteins, nucleic acids, and nanoparticles⁶. Although a variety of functional groups (e.g., hydroxyl, carboxyl, lactone, etc.) are created on the surfaces of NDs during their purification, the conjugation yields of the functional groups are still very low because of the low density of each active chemical group⁷. This results in unstable NDs, which tend to aggregate, limiting further application⁸.

Currently, the most common methods used to functionalize NDs, are covalent conjugation by using copper-free click chemistry⁹, covalent linkage of peptide nucleic acids (PNA)¹⁰, and self-assembled DNA¹¹. The non-covalent wrapping of NDs has also been proposed, including carbohydrate-modified BSA⁴, and HSA¹²coating. However, because these methods are time consuming and inefficient, it is desirable that a simple and generally applicable method can be developed to modify the surfaces of NDs.

Dopamine (DA)¹³, known as a natural neurotransmitter in the brain, was widely used for adhering and functionalizing nanoparticles, such as gold nanoparticles (AuNPs)¹⁴, Fe₂O₃¹⁵, and SiO₂¹⁶. Self-polymerized PDA layers enrich amino and phenolic groups, which can be further utilized to directly reduce metal nanoparticles or to easily immobilize thiol/amine-containing biomolecules on an aqueous solution. This simple approach was recently applied to functionalize NDs by Qin et al. and our laboratory^17,18, although DA derivatives were employed to modify NDs via Click Chemistry in earlier studies^19,20.

Here, we describe a simple PDA-based surface modification method that efficiently functionalizes NDs. By varying the concentration of DA, we can control the thickness of a PDA layer from a few nanometers to tens of nanometers. In addition, the metal nanoparticles are directly reduced and stabilized on the PDA surface without the need for additional toxic reduction agents. The sizes of the silver nanoparticles depend on the initial concentrations of Ag[(NH₃)₂]⁺. This method allows the well-controlled deposition of PDA on the surfaces of NDs and the synthesis of ND conjugated AgNPs, which dramatically extends the functionality of NDs as excellent nano-platforms of catalyst supports, bio-imaging, and bio-sensors.

Protocol

1 . Preparation of Reagents CAUTION: Please read and understand all relevant material safety data sheets (MSDS) before use. Some of the chemicals are toxic and volatile. Please follow special handling procedures and storage requirements. During the experimental procedure, use personal protective equipment, such as gloves, safety glasses, and a lab coat to avoid potential hazards. Preparation of Tris-HCl buffer Dissolve 30.29 g of Tris powder in 100 mL of deionized H2O, ensure that the powder dissolves completely, and then transfer the solution to a 250 mL-volumetric flask. Add deionized H2O to the scale of 250 mL in the volumetric flask to give 1.0 M of Tris buffer. Dilute the 1.0 M Tris buffer 100 times to give 0.01 M Tris buffer and adjust the pH to 8.5 by using 1.0 M HCl standard solution. Use a pH-meter to calibrate the pH value of the 0.01 M Tris-HCl buffer. Preparation of ND suspensions Dilute 100 nm of monocrystalline ND suspensions (1.0 mg/mL) 50 times with the 0.01 M Tris-HCl buffer to give 0.02 mg/mL of ND suspensions. Preparation of dopamine solution Dissolve 20 mg of dopamine hydrochloride in 2.0 mL of 0.01 M Tris-HCl buffer to give 10 mg/mL DA solution. ​NOTE: The DA solution must be freshly prepared and used within 15 min. Preparation of Ag[(NH3)2]OH solution Dissolve 100 mg of AgNO3 solid in 10 mL of deionized H2O to give 10 mg/mL AgNO3 solution. Add 1.0 M ammonium hydroxide (NH3·H2O) dropwise to the AgNO3 solution until yellow precipitate forms, then continue to add the NH3·H2O solution until the precipitation disappears. NOTE: Make the minimum volume required; prepare immediately before use and dispose immediately after use. CAUTION: Add NH3·H2O in fume hood with face shields, gloves, and goggles. 2 . Synthesis PDA Layer on the Surface of NDs (PDA-NDs) Add the freshly prepared DA solution (10 mg/mL) to the ND suspensions to give varied final concentrations of 50, 75, 100 µg/mL of DA. Adjust the total reaction volume to 1.0 mL, transfer it to a 10 mL-test tube, and vigorously stir at 25 °C, in the dark for 12 h. Centrifuge the PDA-NDs solution for 2 h at 16,000 x g, remove the supernatant and wash three times with deionized water for 1 h at 16, 000 x g each time. Re-disperse the PDA-NDs in 200 µL of deionized water with sonication for 30 s. The PDA- coated NDs will be ready for further use. 3 . Reduction of AgNPs on the Surface of PDA-NDs (AgNPs-PDA-NDs) Dilute 40 µL of the pre-synthesized PDA-NDs in Step 2.3 two times with deionized water. Add Ag[(NH3)2]OH solution to give various final concentrations of Ag[(NH3)2]+ (0.08, 0.16, 0.24, 0.40, and 0.60 mg/mL). Adjust the final volume to 100 µL in a 1.5 mL-centrifuge tube by adding deionized water, followed by sonication for 10 min. Centrifuge the AgNPs-PDA-NDs for 15 min at 16,000 x g to remove the free silver ions, discard the supernatant after centrifugation, add 100 µL of deionized water, and wash three times with deionized water at 16,000 x g for 5 min each time. Re-disperse the AgNPs-PDA-NDs in 100 µL of deionizedwater with sonication for 30 s to prepare for further use. 4 . Analysis of PDA-NDs and AgNPs-PDA-NDs Clusters Ultraviolet-visible (UV) spectra Use the UV spectra to monitor the average size distribution of AgNPs on PDA-ND surfaces. Transfer the AgNPs-PDA-NDs samples prepared in Step 3.4 with varied concentrations of Ag[(NH3)2]OH in 1 cm-quartz cuvette and monitor the absorption at a scan wavelength of 250 to 550 nm. Transmission election microscopy (TEM) Place the carbon coated copper grids on a glass slide wrapped with parafilm to keep the grids in place. Insert the glass slide with attached TEM grids into the plasma cleaner. Turn on the plasma cleaner and the vacuum pump. After 5 min, turn on the plasma and discharge the grids with a medium power level for 3 min. Deposit 5 µL of the samples on the carbon film coated Cu-grids for 3 min. Use filter paper to wick off the extra sample from the edge of grid. Then, deposit a drop of deionized water on the grid for 15 s to remove salts, then wick off the water with filter paper. Repeat the washing procedure twice and allow the grid to air dry for further use. Visualize the samples by TEM, typically at 38,000X magnification. Operate at 200 KV.

Representative Results

The formation of PDA layers on ND surfaces were analyzed by TEM (Figure 1). Different thicknesses of PDA layers were observed as higher concentrations of DA led to thicker PDA layers. In addition, after an encapsulating reaction, the color of the NDs solution changed from colorless to dark, while the higher the initial concentration of DA was, the darker the solution became. Fig…

Discussion

This article provides a detailed protocol for the surface functionalization of NDs with self-polymerized DA coating, and the reduction of Ag[(NH₃)₂]⁺ to AgNPs on PDA layers (Figure 3). The strategy is capable of producing various thicknesses of PDA layers by simply changing the concentration of DA. The size of the AgNPs can also be controlled by altering the original concentration of metal ion solution. The TEM image in Figure 1<…

Materials

Nanodiamond	FND Biotech, Inc.	brFND-100	dispersed in water, and used without further purification
Dopamine hydrochloride	Sigma	H8502-25G	prepare freshly
Silver Nitrate	Fisher	S181-25
Ammonium Hydroxide	Fisher	A669S-500	highly toxic
Tris Hydrochloride	Fisher	BP153-500
TEM grid carbon film	Ted Pella	01843-F	300 mesh copper