Nanoporous GaN-Ag Composite Materials Prepared by Metal-Assisted Electroless Etching

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1 Supporting Information for Nanoporous GaN-Ag Composite Materials Prepared by Metal-Assisted Electroless Etching for Direct Laser Desorption-Ionization Mass Spectrometry Bei Nie a,, Barrett K. Duan a and Paul W. Bohn a,b a Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN b Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN Present address: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, People s Republic of China SEM imaging Ag-coated PGaN. In order to study the coverage of Ag NP on PGaN, three samples were prepared and imaged by SEM: electroless Ag deposited on PGaN, electroless Ag deposited on unetched planar GaN, and 20nm Ag nanoparticles dropcast onto PGaN. A crosssection SEM image from the Ag-coated PGaN surfaces is shown in Fig. 1(b), while the Ag NPs deposited onto planar GaN are shown in Fig. S1. In contrast to the isolated Ag particles electrolessly deposited in the nanopores of PGaN, electroless deposition on planar GaN produces indistinct nanoparticle structures, and dropcasting previously-synthesized Ag NPs merely produces a relatively sparse layer on planar or nanoporous substrates. Ag NP density vs. MS signal. Clearly, the surface density of Ag is an important factor in determining the MS signal. Thus, to normalize LDI-MS signal to Ag loading, we estimated the Ag NP density manually from SEM images on the sample types described above. All calculations were based on peak height of the particular mass of interest. Mass spectra were S1

2 acquired using the following parameters: refection mode with 20kV accelerating voltage, acquisition range from 60 to 1600 m/z, spectrum integrated over 200 laser shots, intensity of laser is ~15% of maximal value (estimated laser power is ~30 mj/cm -2 ). Utilizing Ag nanoparticles in isolation as LDI substrate, the estimated Ag NP density on dropcast surfaces is ~36 particles µm -2 as determined from the SEM micrograph in Fig. S1. In contrast, electroless deposition produces an average of ~110 countable particles µm -2, a number which does not include the Ag particles deposited on the interior surfaces, as shown in Fig. 1(b). Hence, scaling the 20x increase to the Ag particle density indicates that a factor of 3 increase results from the increase in areal density, while the remaining factor of ~6.5 derives from Ag deposited on the interior nanopore surfaces. These results utilizing cholesterol as a model compound were augmented with additional experiments utilizing adenine. Adenine exhibits very little signal on Ag NP substrates, while a peak [AgAd] + at m/z = 242/244 and a triplet set of [Ag 2 Ad-H] + at m/z 348/350/352 are observed in the LDI-MS spectrum acquired from Ag-PGaN, Fig. 5, main text. These latter results suggest that Ag-decorated porous networks can effectively ionize relatively inactive compounds. In addition, the background from Ag + and associated products remains relatively constant, while the ion signals from analytes significantly increase. Using the same calculation method described above, the ratios of background ion counts are found to be: Ag + PGaN/Ag Ag only = 1.87; Ag 2 PGaN /Ag 2 Ag only = 0.76; Ag 3 PGaN /Ag 3 Ag only = MS analysis of R6G. To verify the MS spectra from Ag NP surfaces, an ethanolic R6G HCl solution (1µM) was analyzed by both regular MALDI and electrospray mass spectrometry. In MALDI, a drop (1µL ) of 1µM R6G Cl was mixed into 1µL of 10mg/mL DHB methanolic solution. The resulting mixture was then deposited on an ITO-glass slide and allowed to dry entirely under ambient conditions. The MS spectra were acquired on a Bruker- S2

3 Dalton Autoflex III MALDI-TOF-TOF equipped with a frequency-tripled Nd:YAG Laser producing 355nm pulses at 100 Hz. The mass spectrometer was set to refection mode with 20kV accelerating voltage, acquisition range from 60 to 1600 m/z, spectrum integrated over 200 laser shots, laser intensity ~25% of maximal value (estimated laser fluence is ~30 mj/cm -2 ). The resulting data were recorded and analyzed by software (Autoflex Analysis) provided by Bruker- Dalton. All data were digitally exported and reconstructed using Igor Pro 6.1 (Lake Oswego, OR). The electrospray MS analysis was performed on a Thermo LTQ XL linear ion trap mass spectrometer. The sample was infused via syringe pump at a flow rate of 3µL/min. The mass spectrometer was set to a source voltage 5.0kV; source current 3.2µA; sheath gas flow rate (arb.) 8.0; capillary temperature ; capillary voltage 47.96; full scan range m/z; AGC on and micro scan count 3. Figure S3 illustrates the resulting spectra from MALDI and ESI-MS, respectively, indicating the existence of [R6G+H] + (m/z 443.5), i.e. the protonated free base, after dissociation of the Cl - counter ion. S3

4 500nm Figure S1. SEM micrograph of a typical dropcast Ag nanoparticle substrate, showing both isolated Ag NPs as well as aggregated structures. S4

5 Figure S2. LDI time-of-flight mass spectrum of 10 picomoles of cholesterol dropcast onto Ag-coated anodic aluminum oxide membrane with 100 nm pores. The coating process utilizes the same electroless deposition procedures used to produce Ag nanostructures in PGaN. S5

6 Figure S3. (a) ESI-MS analysis of 1µM R6G; (b) MALDI mass spectrum of 10 picomoles R6G co-deposited with 10 mg/ml DHB matrix. Both spectra are dominated by the cation at m/z 443.5, representing protonated R6G free base. S6