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Synthesis of Platinum Nanoparticles and the Study of Their Efficiency As Substrates for Laser Desorption/Ionization (LDI) of Model Analytes

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Synthesis of Platinum Nanoparticles and the Study of Their Efficiency As Substrates for Laser Desorption/Ionization (LDI) of Model Analytes Platinum nanoparticles synthesized by laser ablation in water and their use as substrates for the soft laser desorption/ionization of polymers and peptides Maite Cueto1*, F. Gámez2, A. R. Hortal2, P. Hurtado2, B. Martínez-Haya2 M. Sanz3, M. Oujja3, M. Castillejo3 1 Instituto de Estructura de la Materia CSIC, Madrid (Spain) 2 Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Seville (Spain) 3 Instituto de Química Física Rocasolano CSIC , Madrid (Spain) *E-mail: [email protected] Summary - The object of this work is the synthesis of platinum nanoparticles and the study of their efficiency as substrates for laser desorption/ionization (LDI) of model analytes. - Platinum stands out among the noble metals as a nanostructured assisted LDI (NALDI) active substrate because of its low heat conductivity and high melting temperature [1]. - We have synthesized Platinum nanoparticles by high energy pulsed laser ablation of Pt foil in aqueous solution. - Different Nd:YAG laser wavelengths (266, 532, 1064 nm) and stabilizing agents (PEG, PVA, citrate) have been used to control the nanoparticle size and crystallinity [2,3]. - Pt nanoparticles with single and bimodal size distributions (ranging 2-30 nm) and of spherical and rod-like shape have been obtained. - The nanoparticles have been tested as active substrates for the soft laser desorption/ionization (NALDI-MS) of a synthetic polymer (polyethylenglycol PEG600) and a peptide (Angiotensin I). - The NALDI-MS experiments were performed in positive-ion mode with a commercial time-of-flight mass spectrometer (UltrafleXtreme, Bruker, 355nm laser source) and demonstrate that Platinum nanoparticles provide a good sensitivity for the detection of these model analytes. - The best overall NALDI-MS performance was found for nanoparticles with sizes 2-10 nm, produced with 1064 nm laser pulses, in water or with a light stabilizing agent such as citrate. Experimental Details Size distributions Nanoparticles average size (nm) crystallinity Size distributions Nanoparticles average size (nm) Crystallinity Production of nanoparticles by Laser ablation in water HRTEM Q-switched Nd:YAG laser (Quantel, Brilliant B),6 ns FWHM, operating at 10 Hz during 15 min. Laser fluences of 7, 3.5, and 1.5 Jcm-2, close to the ablation threshold, were employed at the respective wavelengths of 1064, 532 and 266 nm. - Bimodal size distribution: Small size component is presumably associated with a thermal vaporization mechanism and large size component to an explosive ablation mechanism, enhanced at the longer ablation wavelengths. - Short wavelength ablation yields crystalline NPs of small size and large crystalline nanorods (10x70 nm2), especially when PEG and PVA are employed. - The smallest NPs were obtained in citrate solutions due to the more efficient coating of the ablated material. PEG-Np18B-Na 5-7-5_35% PEG-Np17B-Na 5-7-5_35% PEG-Np45B-Na 5-2-5_35% 20000 Sample plate 1 20000 Sample plate 1 13 Sample plate 1 12 13 14 6000 13 Sample plate 2 Sample plate 2 Sample plate 2 12 14 12 Sample plate 3 14 Sample plate 3 Sample plate 3 15 Average 11 Average 11 Average 15000 15 15000 11 16 + PEG-Na 15 + 4000 + NALDI Experiments PEG-Na . + PEG-Na 10 16 PEG-K 10 17 . + . + PEG-K 10000 16 PEG-K 10000 10 Intensity/ a.u. Intensity/ . Intensity/ a.u. Intensity/ . 17 18 a.u. Intensity/ 9 17 9 . . . . 2000 9 . . 8 19 5000 8 . 18 . 5000 . 18 . 8 . 19 . . . 19 . . 7 . 7 . 0 0 0 200 300 400 500 600 700 800 900 200 300 400 500 600 700 800 900 200 400 600 800 m/z m/z m/z NPs synthesized in water : 18B at 1064 nm NPs 17B at 532 nm NPs 45B at 266 nm Ang-Np25B-Na 5-5-5 45% Ang-Np9B-Na 5-5-5 45% + Ang-Np50B-Na 5-2-5 45% 25000 [Ang-Na] 10000 8000 Sample plate 1 Sample plate 1 + + [Ang-Na] Sample plate 1 Sample plate 2 [Ang-Na] Sample plate 2 Sample plate 2 20000 Sample plate 3 Sample plate 3 Average Sample plate 3 Average Average 15000 [Ang+2Na-H]+ 5000 4000 [Ang+2Na-H]+ 10000 TOF-MS (UltrafleXtreme, Bruker), Nd:YAG [Ang+2Na-H]+ Intensity/ a.u. Intensity/ Intensity/ a.u. Intensity/ Intensity/ a.u. Intensity/ [Ang-H]+ laser at 355 nm, operating at 1 kHz. Mass + 5000 + [Ang-K] + [Ang-K] + spectra averaged over 2000 laser shots. [Ang-H] [Ang-H] [Ang-K]+ 0 0 0 1300 1320 1340 1300 1320 1340 1300 1320 1340 m/z m/z m/z NPs synthesized in citrate: 25B at 1064 nm NPs 9B at 532 nm NPs50B at 266 nm Conclusions - The best NALDI-MS performances were obtained with Platinum Nanoparticles produced at 1064 nm in pure water and citrate as stabilizing agent. This result is attributed, on one hand, to the adequate size of the nanoparticles for the analyte adsorption and for a more efficient analyte desorption due to plasmonic excitacion. On the other hand, light stabilizing agents such as citrate provide a homogeneous and close interaction between the nanoparticle and the analyte better than polymers such as PVA, that act as molecular spacers. [1] T. Yonezawa, H. Kawasaki, A. Tarui, T. Watanabe, R. Arakawa, T. Shimada, F. Mafuné, Analytical Sciences, 25 (3), 339-347 (2009). References [2] W. T. Nichols, T. Sasaki, N. Koshizaki, Journal of Applied Physics, 100, 114911-114913 (2006). [3] M. Cueto, M. Sanz, M. Oujja, F. Gámez, B. Martínez-Haya, M. Castillejo, Journal of Physical Chemistry C, 115, 22217–22224 (2011). Acknowledgements Funding from the FEDER-Andalucia2007-2013 through projects P07-FQM-02600 and P09-FQM-4938 . Funding from MICINN under Projects CTQ2010-15680 and CONSOLIDER CSD2007-00058, and Programa Geomateriales (CAM, S2009/Mat-1629). We thank GEMPPO at the IEM-CSIC for the use of the HRTEM and CITIUS from Universidad de Sevilla for the use of the TEM equipment. .
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