Bio-inspired Polydopamine Surface Modification of Nanodiamonds and Its Reduction of Silver Nanoparticles

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

This method can help answer key questions about effective surface functionalization of nanodiamond, which have broad applications in material science and biomedicine. This method can be used for nanodiamonds. It can also be applied for other materials, such as metal nanoparticles, magnetic nanoparticles, or surfaces that need an active biopolymer coating.

In this method, nanodiamonds are funtionalized with a coating of polydopamine, a universal adhesive. The thickness of the PDA layer is well controlled by varying the concentration of dopamine. To begin, dissolve 30.29 grams of tris HCl powder in 100 microliters of deionized water.

Transfer the solution to a 250 milliliter volumetric flask. Fill the flask to the line with deionized water, and mix it to obtain a one molar tris HCl buffer. From this stock solution, prepare 20 milliliters of 0.1 molar tris HCl buffer by serial dilution.

While monitoring the buffer with a pH meter, adjust the pH to 8.5 using one molar hydrochloric acid. Next, dilute 0.02 milliliters of a one milligram per milliliter suspension of 100 nanometer monocrystalline nanodiamonds to one milliliter with the pH 8.5 tris buffer. Stir the mixture for 10 minutes to obtain a 0.02 milligram per milliliter nanodiamond suspension.

Then, dissolve 20 milligrams of dopamine hydrochloride in two milliliters of pH 8.5 tris buffer, by vortexing for 30 seconds to obtain a buffered 10 milligram per milliliter dopamine hydrochloride solution. Add five, 7.5, or 10 microliters of the freshly prepared dopamine solution to the nanodiamond solution, depending on whether a final dopamine hydrochloride concentration of 50, 75, or 100 milligram per milliliter is desired. After adjusting the reaction volume, vigorously stir the mixture at 25 degrees Celsius for 12 hours in the dark.

Then, transfer the suspension of polydopamine-coated nanodiamonds to a 1.5 milliliter centrifuge tube, and centrifuge it at 16, 000 g for two hours. Remove the supernatant and wash the nanodiamonds three times with one milliliter portions of deionized water at 16, 000 g for one hour each time. Then add 200 microliters of deionized water to the washed solids, and sonicate the mixture for 30 seconds to redisperse the polydopamine-coated nanodiamonds.

Serially dilute 40 microliters of a suspension of polydopamine-coated nanodiamonds twice with deionized water. Then, dissolve 100 milligrams of silver nitrate in 10 milliliters of deionized water by vortexing. In a fume hood, add one molar aqueous ammonia to the silver nitrate solution, dropwise, until a yellow precipitate forms, periodically shaking the solution.

Continue adding ammonia until the precipitate disappears to obtain a solution of diamine silver hydroxide. Immediately add either 4.3 or 6.4 microliters of the diamine silver solution to 40 microliters of the diluted nanodiamond dispersion, for a final concentration of 0.4 or 0.6 milligrams per milliliter respectively. After this, adjust the volume to 100 microliters with deionized water.

Sonicate the mixture for 10 minutes. Then, centrifuge the dispersion for 15 minutes at 16, 000 g to remove free silver ions. Discard the supernatant and wash the silver nanoparticle-decorated polydopamine-coated nanodiamonds by centrifuging them three times in 100 microliter portions of deionized water at 16, 000 g for five minutes each time.

Add 100 microliters of deionized water to the silver nanoparticle-decorated nanodiamonds and redisperse them by sonication for 30 seconds. Characterize the nanodiamonds with UV-Vis spectroscopy scanning from 250 to 550 nanometers. Next, deposit five microliters of the silver nanoparticle-decorated nanodiamonds on plasma-cleaned carbon-coated copper grids, and let them sit for three minutes.

Then, wick away excess solution with filter paper. Wash each grid three times by applying a drop of deionized water, letting it sit for 15 seconds, and then wicking it away with filter paper. Let the grids air dry before visualizing the samples with transmission electron microscopy.

The uncoated nanodiamonds tended to form microclusters and aggregates, while polydopamine-coated nanodiamonds formed good dispersions. Higher dopamine concentrations resulted in the formation of thicker polydopamine layers in the nanodiamond surfaces. The uncoated nanodiamond dispersion was clear and colorless.

Upon coating the nanodiamonds with a five nanometer thick polydopamine layer, the dispersion appeared cloudy and brown. The dispersion appearance became progressively darker with thicker polydopamine coatings. The reduction of diamine silver on nanodiamonds coated with a 15 nanometer thick polydopamine layer was most successful when the diamine silver hydroxide concentration was 0.4 to 0.6 milligrams per milliliter.

The nanodiamonds formed at lower concentrations were too small to be studied effectively. The maximum absorbance values indicated that the nanoparticles formed from the 0.4 to 0.6 milligram per milliliter solutions had diameters of roughly 20 and 30 nanometers respectively. TEM showed that the silver nanoparticles generated from the 0.4 milligram per milliliter diamine silver solution were about 24 nanometers wide, while the nanoparticles generated from the 0.6 milligram per milliliter solution were about 28 nanometers wide.

The number of nanoparticles on the nanodiamond surfaces was also greater at the higher diamine silver concentration. After using this procedure, well-dispersed nanodiamonds with a controllable PDA thickness were formed. This technique paves the way for researchers to explore nanodiamond applications for catalyst, biosensors, and nanocarriers.

Without using any additional reducing agent, the PDA-assisted materialization process can induce the formation of silver nanoparticles upon the reduction of metal precursors and immobilize them on the PDA-coated surface. In addition, the PDA layers contain open functional groups which can be further utilized to conjugate serial and amine-modified biomolecules.