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Laser Desorption Ionization Mass Spectrometry by Using Surface Plasmon Excitation on Gold Nanoparticle K. Shibamoto,* K. Sakata, K. Nagoshi, and T. Korenaga Department of Chemistry, Tokyo Metropolitan UniVersity, Minami-Osawa 1-1, Hachioji, Tokyo, 192-0397 Japan ReceiVed: March 5, 2009; ReVised Manuscript ReceiVed: August 10, 2009
We show development of a laser desorption/ionization (LDI) method with ultrahigh sensitivity by adding gold nanoparticles with a diameter of several tens of nanometers into a sample solution. In this paper, we succeeded in detection of an ultratrace amount of sample molecules, which is less than several hundred zeptomoles. This result suggest that the charge interaction based on surface plasmon (SP) excitation is very effective for ultrahigh sensitivity in LDI method. Introduction Property of a metal nanoparticle depends on diameter, shape, and metal species to its large specific surface area. Especially, a metal nanoparticle with a diameter of several tens of nanometers can effectively absorb laser energy in the specific wavelength region via surface plasmon (SP) excitation. Because the stored energy in the metal nanoparticle is localized on its surface, this SP excitation is used for various applications such as spectroscopic analysis1-5 (Raman spectroscopy, fluorescence spectroscopy, and infrared spectroscopy) and nanoimaging,6 among others. The matrix-assisted LDI (MALDI) method developed by K. Tanaka,7,8 who received the 2002 Nobel Prize in Chemistry, has a very useful analytical performance. Because this MALDI method had been improved remarkably in dissociation of target molecules compared with conventional LDI methods,9,10 the use of the MALDI method has spread rapidly all over the world as a very effective analysis method.11-14 In this MALDI method, sample molecules are usually added to excess matrix molecules, which are low-mass organic molecules. These molecules have three major roles to measure with high analytical performance. The first is to reduce molecular interactions between sample molecules by surrounding an isolated sample molecule, because the strong molecular interaction in LDI measurement causes many fragmentation peaks and makes detection of nondissociated sample molecules ions difficult. The second is that these molecules absorb the laser energy, and their stored energy is used for desorption of sample and matrix molecules. The third is that these molecules become a proton [H+] source to ionize sample molecules as [M+H]+. Therefore, the user’s choice of matrix molecule, which strongly depends on compatibility with the sample molecule, determines the analytical performance of the MALDI method. However, there is no “golden matrix” which has similar compatibility with all sample molecules. In other words, this negative effect means that MALDI cannot be a powerful analysis tool for unknown sample molecules. Furthermore, other negative effects related to analytical performance exist. The mixture of sample molecules and excess matrix molecule has large heterogeneities of not only concentra* Corresponding author. E-mail:
[email protected], Phone: +8142-677-2529, Fax: +81-42-677-2525, Tokyo Metropolitan University Department of Chemistry.
tion but also morphology. These heterogeneities cause reproducibility to be too low to analyze quantitatively. In the past decade, various surface-assisted LDI (SALDI) methods have attracted much attention because these SALDI methods do not need the addition of matrix molecules. As SALDI substrates, porous silicon,15-18 other materials with roughened surface,19,20 pyroelectric ceramic plate,21 surface modified silicon with organic molecules for trapping target molecules,22,23 and nanoparticle surfaces,24-28 among others, have been proposed. However, concerning practical utility, the remarkably lower sensitivity of the most SALDI method compared to that of the MALDI method, except in a few reports of SALDI methods,29 is more serious than the above-mentioned negative effect due to the addition of excess matrix molecule in the MALDI method. The remarkably low sensitivity may be caused by the assumption that the ionization mechanisms in the SALDI methods are similar to the ionization mechanisms mainly based on rapid thermal energy supply in the MALDI method. Actually, the ionization mechanism on the abovementioned SALDI substrates is very difficult to explain by only the rapid thermal energy supply. We should consider that these SALDI substrates give not only thermal energy but also some surface effects from their surface to sample molecules. Understanding these additional surface effects leads to improving their analytical performance. In other words, these SALDI methods using specific surface effects may exceed the MALDI method in analytical performance. We focused on charge interaction between metal surface and sample molecules as a new surface effect. It is expected that the charge interaction does not supply too much energy to dissociate sample molecules because the charge transfer between the surface and sample molecules is “smooth” and differs from high-energy electron collisions used in the electron impact (EI) method.30 We introduced highly efficient charge interaction caused by surface plasmon (SP) excitation into the SALDI method by using bare gold nanoparticles with a diameter of several tens of nanometers. Although a few LDI methods used nanoparticles such as bare Ag nanoparticles,39 and smaller gold nanoparticles40,41 were reported by several groups, the nanoparticles in these reports were used as strong energy absorbers. The SP excitation is explained below. When a specific metal surface with nanoscale roughness, such as metal nanoparticles and surface-roughened thin metal films, is irradiated with a specific wavelength laser, almost all of the laser energy is
10.1021/jp9020432 CCC: $40.75 2009 American Chemical Society Published on Web 09/17/2009
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absorbed resonantly. Then, a great number of excited electrons in the metal are generated and are localized on the metal surface. Therefore, these excited electrons are densified very much and induces enormous enhanced electromagnetic field in nanospace on the surface. Most of the above-mentioned applications using the SP excitation, such as nanoscale fabrication, ultrasensitive laser fluorescence, and ultrasensitive infrared spectrometry, focus on the enormous enhanced electromagnetic field. On the other hand, it is known that ultrasensitive Raman spectrometry using the surface-enhanced Raman scattering (SERS) effect1-3 is attributed to not only the enormous enhanced electromagnetic field but also the charge interaction based on the dense excited electrons on the metal surface between the surface and the chemically adsorbed molecules. Although there are a few reports of the LDI method as an application of SP excitation,31,32 these methods are similar to the MALDI method because a large amount of metal ions of SP excitation material caused by ablation are detected in these methods. In several of these reports, aggregations of metal nanoparticles are used as SP excitation species because it is difficult to use isolated metal nanoparticles. However, the use of aggregations of metal nanoparticles makes it difficult to discuss active specific surface area and active volume working the SP excitation effect on their surface in detail. On the other hand, the ionization mechanism of our proposed new SP LDI method may be clearer than that of these methods because we used isolated nanoparticles. Furthermore, we focused not only on thermal energy in our method, but also the charge interaction based on SP excitation. Because the charge interaction could supply some energy to sample molecules directly, it is expected that the charge interaction is quite effective to develop a new SALDI method with ultrahigh sensitivity, and then our method may lead to detection of ultratrace amounts of target analyte such as biomolecules in single cell and environmental molecules.
excitation of the gold nanoparticles. The laser power was controlled with a variable reflectivity neutral density filter and was set to range from 6.0 µJ/pulse to 40 µJ/pulse. The diameter of the laser spot size was 5 mm. Therefore, the energy fluence ranged from 30 µJ/cm2 (300 nJ/mm2) to 200 µJ/cm2 (2 nJ/mm2). The pulses were focused with a 300 mm spherical plano convex lens of fused silica and were obliquely irradiated onto the ionization substrate at the position of 270 mm from the lens. In our system, the irradiating angle was 45° and the actual energy fluence on the ionization substrate can be estimated about from 2.2 mJ/cm2 (22 µJ/mm2) to 14 mJ/cm2 (140 µJ/mm2). Sample Molecules and Solvents. As sample molecules, crystal violet (CV), N-acetyltetraose (N-AT), and angiotensin II molecules (Ag II), among others, were used. CV and N-AT sample molecules were dissolved in aqueous solution. Ag II sample molecules were dissolved in a mixed solution of ethanol and acetonitrile (3:7). The standard amount of these sample molecules was set equal to the amount that can cover the gold nanoparticle surface as a monolayer. The amount of adsorbed molecules was determined by estimating the surface area of the gold nanoparticle and the average adsorption area of each sample molecule. We show an example of estimation for a CV molecule. The CV molecule, which is one of the triphenyl dye molecules, has a structure with a carbon in the center and three benzene rings at each apex of a triangle. Therefore, the CV molecule is roughly regarded as a circle with a diameter of 1 nm, and the number density of CV molecule per unit area is estimated at 1.3 × 1014 molecules/cm2. Preparation of Ionization Substrates. Ionization substrates were prepared by dropping a mixed aqueous solution of the sample solution and the gold nanoparticle solution onto an n-type (100) Si wafer (Nilaco). While controlling the ratio of sample solution and gold nanoparticle solution, the amount of sample molecule was determined.
Experimental Section
Results and Discussion
Gold Nanoparticles in Aqueous Solution. We used gold nanoparticles in aqueous solution without any surface chemical modification, which were purchased from the British Biocell International (BBI), as nanoparticles for the SP excitation. The sizes of the gold nanoparticles in this paper were 5, 50, 60, 80, 100, and 250 nm, with about 8% standard deviation. The concentrations of the gold nanoparticle aqueous solution were dilute enough to avoid aggregation (2.6 × 1010/mL for 60 nm nanoparticles). Because gold is very stable to chemical reagents and its surface is very hard to oxidize, the charge interaction would work without being disturbed by the surface oxidation. In the case of using an isolated gold nanoparticle, laser irradiating around a wavelength of 520 nm induces strong SP excitation. The efficiency of the SP excitation depends on the diameter of the nanoparticles. Characterization of Gold Nanoparticles. Gold nanoparticles were characterized by scanning electron microscopy (SEM; KEYENCE VK-9700). Their SP excitation efficiencies were estimated from the absorption efficiency by absorbance determination (SHIMADZU UV-1600). Laser Desorption/Ionization Mass Spectrometry. LDI measurements in this paper were performed in positive ionization mode by our original LDI equipment, which is a linear time-of-flight mass spectrometer with delayed extraction. We used a Nd:YAG pulse laser as a light source in our LDI measurement. The pulse train wavelength was 1064 nm, with a repetition rate of 10 Hz and a pulse width of 7 ns. The pulses were frequency doubled to a wavelength of 532 nm for the SP
Absorption spectra of all gold nanoparticle aqueous solution are shown in Figure 1. A large peak in the region from 500 to 600 nm corresponds to the surface plasmon excitation. Absorption spectrum of smaller gold nanoparticle (5 nm) had a high baseline caused by aggregation. This broader component does not contribute to SP excitation. Therefore, we should subtract the baseline from the absorption spectrum for rough estimation of SP excitation efficiency, as shown in Figure 1. 50, 60, 80, and 100 nm gold nanoparticle aqueous solutions had very large SP excitation efficiencies at the wavelength of 532 nm, which was used as a LDI laser source. These results mean that gold nanoparticles whose diameters are from 50 to 100 nm strongly induce the SP excitation. On the other hand, 5 and 250 nm gold nanoparticle aqueous solutions had small SP excitation efficiencies at the same wavelength. An SEM image of the prepared ionization substrate surface by using a 60 nm gold nanoparticle aqueous solution is shown in Figure 2. Large aggregations of 60 nm gold nanoparticles were not generated in our preparation condition of ionization substrates, although many small aggregations were generated. A large aggregation gives a huge SP excitation efficiency due to the aggregation effect. However, the large aggregation would indicate some serious defects for our LDI method. First, the actual effective surface area of gold nanoparticles cannot be determined. Second, trapped sample molecules in the large aggregation are hard to desorb from the aggregation. Third, the ionization mechanism gets very complex. Aggregation of gold nanoparticles makes any discussions of particle diameter hard, because large aggregation
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Figure 1. Absorption spectra (top) and corrected absorption spectra (bottom) of gold nanoparticle aqueous solution (5, 50, 60, 80, 100, and 250 nm).
Figure 3. LDI mass spectra of mixed solutions of 1 µL aqueous solution of N-AT molecules (1 fmol) and (a) 1 µL aqueous solution without gold nanoparticles, (b) 1 µL aqueous solution of 5 nm gold nanoparticle (4.5 × 107 particles), and (c) 1 µL aqueous solution of 50 nm gold nanoparticle (4.5 × 105 particles).
Figure 2. SEM image of 60 nm gold nanoparticles on a Si plate. Its magnification is 60 000×. The length of the scale bar is 100 nm.
and aerosol consisting of small nanoparticles are similar to a surface-roughened large particle in optical responsivity, and its heterogeneous SP excitation efficiency according to location strongly depends on the diameter of the large aggregation and aerosol. For these reasons, only ionization substrates without large aggregations were used in our all measurements. Figure 3 shows LDI mass spectra of mixed solutions of 1 µL aqueous solution of N-AT molecules (1 fmol) and (a) 1 µL aqueous solution without gold nanoparticles, (b) 1 µL aqueous solution of 5 nm gold nanoparticle (4.5 × 107 particles), and (c) 1 µL aqueous solution of 50 nm gold nanoparticle (4.5 × 105 particles). These mass spectra were generated by averaging data obtained from 512 laser shots. Although the dropped area
of sample solution for each mixed solution on the Si surface was 12 mm2, the laser spot area in our LDI measurement system was 0.28 mm2. Therefore, the amount of sample molecule per laser spot was about 20 amol. N-AT sample molecules without gold nanoparticles were not detected as shown in Figure 3a. This result means that the amounts of N-AT ions and its fragment ions caused by multiphoton ionization process are below the detection limit in our LDI measurement system, and therefore, the contribution of multiphoton ionization can be eliminated in our discussion. On the other hand, even though N-AT molecules were less than 20 amol in the laser spot of our LDI measurement system, a peak of dewatered biose ions (m/z ) 389), which corresponds to fragment ion of N-AT molecules,33 was strongly detected in the N-AT sample mixed with 50 nm gold nanoparticles as shown in Figure 3c and weakly detected in the N-AT sample mixed with 5 nm gold nanoparticles as shown in Figure 3b. Other peaks at m/z ) 23, 39, and around 100 in these mass spectra were attributed to Na+ ion, K+ ion, and fragment ions of the N-AT molecule. The quantity of N-AT ions was very small, so we proceed to a discussion by using the quantity of the dewatered biose ion, which was large enough. The intensity of the dewatered biose ions in the 50 nm sample was more than 3 times larger than that in the 5 nm sample. Because the total surface areas of gold nanoparticles
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Figure 4. Triangle marks (right axis) and circle marks (left axis) show the dependence of dewatered biose (m/z ) 389) ion intensity from the N-AT samples mixed with gold nanoparticle aqueous solution and SP excitation efficiency estimated from Figure 1 on the diameter of the gold nanoparticle (5, 50, 60, 80, 100, and 250 nm).
in both samples were comparable, it is considered that the difference in the peak intensity of the dewatered biose ions was caused by the difference in the optical responsivity of each gold nanoparticle surface. The result of Figure 1 shows that this difference in optical responsivity was caused by the difference in the efficiency of the SP excitation of gold nanoparticles at 532 nm. From our estimation, the SP excitation efficiency of 50 nm gold nanoparticle was more than 3 times higher than that of 5 nm gold nanoparticle. Therefore, 50 nm gold nanoparticle strongly induced the SP excitation, and then, a large proportion of the irradiating laser energy was spent to excite the collective oscillation of free electrons in the surface region of the gold nanoparticles. Its surface has a unique field with a huge local density of excited electrons. In SERS studies, it has been reported that the field gives two effects, an enhanced electromagnetic (EM) effect in the surface region and a charge transfer (CT) effect between the surface and directly adsorbed sample molecules, to sample molecules adsorbed onto the surface.34-37 In our LDI measurement system, it is suggested that the N-AT sample molecules directly adsorbed onto the gold nanoparticle surface received these two effects, and then, the ionization efficiency of the sample molecule was dramatically improved. In order to consider the charge interaction based on the SP excitation between the gold nanoparticle surface and adsorbed sample molecules, we demonstrated the following experiments. By using the estimated dependence of the diameter of gold nanoparticles on the SP excitation efficiency at 532 nm, we compared this with a correlation between the dewatered biose ion intensity in the N-AT sample and diameters of gold nanoparticles and the dependence, as shown in Figure 4. In the region from 5 to 60 nm, the N-AT sample with larger gold nanoparticles gave the larger dewatered biose ion intensity. This dependence obtained from our results did not agree with ref 31. The reason comes from the fact that in ref 31 aerosol aggregation of gold nanoparticle (5 nm, 19 and 44 nm) with a large diameter of several 100 nm was used as a SP excitation species and its SP effect is affected by both actual surface area and its diameter. However, in the further large region, the sample gave smaller mass signal intensity. On the other hand, our result agreed with the dependence of estimated SP excitation efficiency on the diameter of gold nanoparticles shown in Figure 4 and suggested that the efficiency of the SP excitation is strongly associated with the efficiency of the ionization in the LDI method. Additionally, this dependence was remarkably similar to the dependence of the SERS enhancement factor on the diameter of gold nanoparticles reported by M. Kitajima and his co-workers.38 These results mean that two effects caused by the SP excitation relate to the enhancement of mass signal
Figure 5. LDI mass spectra for various amount of the N-AT sample (100 fmol, 10 fmol, 1 fmol, 100 amol, and 10 amol in descending order) with 50 nm gold nanoparticles (4.5 × 105 particles).
intensity. Furthermore, the author of this paper had also succeeded in direct observation of the charge interaction between a gold surface and the absorbing sample molecules by using an ultrafast spectroscopic method with femtosecond time resolution, which can monitor directly the ultrafast charge interaction.34-37 Therefore, it is considered that the charge interaction based on the SP excitation between gold nanoparticles and the N-AT sample molecules may contribute to the effective desorption/ ionization mechanism in our system. In order to discuss the charge interaction in more detail, a dependence of the dewatered biose ion intensity in the N-AT sample with 50 nm gold nanoparticles on the amount of N-AT sample molecules was measured. This result is shown in Figure 5. We succeeded in detecting dewatered biose ions from several 10 amol samples of N-AT molecules. This result shows a significant improvement of detection limit in conventional LDI methods including the MALDI method. The dewatered biose ion intensity increased in order of the 10 amol sample, 100 amol sample, and 1 fmol sample until the amount of the N-AT molecules reached the amount covering the gold nanoparticle surface by a monolayer, although the increasing ratio of the dewatered biose ion intensity was lower than that of the amount of the N-AT molecules. Because the irradiating laser power was constant in our system, the number of excited electrons induced on the gold nanoparticle surface was constant for each sample. Therefore, the number of excited electrons per one N-AT molecule was larger for the sample containing the smaller amount of N-AT molecules, and it is considered that the ionization efficiency of N-AT molecules, which received the charge interaction based on the SP excitation, increased in the order of the 1 fmol sample, the 100 amol sample, and the 10 amol sample. On the other hand, the dewatered biose ion intensity decreased in the order of the 1 fmol sample, 10 fmol sample, and 100 fmol sample. Because the enhanced electromagnetic field affects the piled N-AT molecules included in several 10 nm distance from the gold nanoparticle surface, all N-AT molecules in our system should receive some influence from the enhanced electromagnetic field and then the dewatered biose ion intensity should increase as the amount of the N-AT molecules increases. However, our results were unpredictable and cannot be explained by only a contribution of the enhanced electromagnetic field. Therefore, this behavior suggests that the piled N-AT molecules except in
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Figure 6. SP assisted LDI mass spectrum of CV molecules whose amount is about 500 zmol.
the first layer gave some inhibition of ionization of the N-AT molecules in the first layer. We discuss this inhibition from three viewpoints. From the first viewpoint of the ionization mechanism, it is considered that the N-AT molecules except in the first layer receive only the enhanced electromagnetic field effect, because only those in the first layer can also receive the charge interaction effect, and further, these molecules prevent those except in the first layer from receiving the charge interaction effect. This means that the ionization mechanism of the N-AT molecules in the first layer differs from that of the N-AT molecules except in the first layer. This discussion in this viewpoint cannot deny our results. From the second viewpoint of energy supply, it is considered that some energy, which the N-AT molecules in the first layer receive via the charge interaction, diffuses to other piled N-AT molecules and therefore the energy supply efficiency per one N-AT molecule decreases. Piled molecules may act as an energy buffer. From the third viewpoint of ablation direction, it is considered that the piled N-AT molecules except in the first layer act as a cover for the N-AT molecules in the first layer, and the ratio of ionized N-AT molecules which can reach the detector in our LDI measurement system decreases. For the above reasons, our results suggest that the charge interaction based on the SP excitation between the gold nanoparticle surface and adsorbed molecules assists in ultrahigh ionization efficiency of adsorbed molecules directly onto the gold nanoparticle surface. Therefore, our method is a strong analytical method for the first layered molecules on the surface because these molecules receive the two effects based on the SP excitation without inhibition of piled molecules. In order to confirm the contribution of this charge interaction, crystal violet (CV) dye molecules adsorbed onto our SALDI substrate, whose amount is 500 zmol ()1 pM × 0.5 µL), were measured with our LDI system. K. Shibamoto and co-workers reported that the CV molecule efficiently receives the charge interaction from SP-active gold surfaces in their SERS study.34-37 The result of LDI measurement was shown in Figure 6. Although the amount of CV molecules was significantly small, the LDI signal intensity of nondissociated CV molecules (m/z ) 372) were detected more strongly than other sample molecules such as the N-AT molecule, Ag II molecule, and other dye molecules in the same experimental condition. This result means that the CV molecule receives a strong effect for effective ionization by the charge interaction based on the SP excitation. Furthermore, we attempted to detect other molecules such as the Ag II molecule and malachite green dye molecule, among others. We show a mass spectrum of Ag II in comparison to the conventional LDI spectrum of Ag II in Figure 7. The sample quantity of Ag II was several femtomole (monolayer). Although
Figure 7. SALDI SP assisted (top) and LDI without gold nanoparticles (bottom) mass spectrum of Ag II molecules. Extended mass spectra are shown in an inset.
irradiation laser power in the case of the Ag II was stronger than that in the case of CV molecule, we succeeded in detection of a small amount of Ag II ions with several fragmentation peaks of Ag II. All of these sample molecules in our LDI system using gold nanoparticles as SP excitation species were able to be detected more easily than in the MALDI method, and the detection limit of our method was greater than that of the MALDI method by one or more orders of magnetide. From these results, our SALDI method shows useful analytical performance for various kinds of analyte molecules. In summary, we succeeded in detection of a significantly small amount of analyte molecules, less than several 100 zmol, by utilizing the ultraeffective charge interaction based on SP excitation as the energy source for effective ionization. The most important point in this high analytical performance is that we focused not only on the thermal energy supply reported as a main ionization energy source in conventional LDI methods, but also on the direct energy supply induced by charge interaction and enhanced electromagnetic field based on SP excitation. It is expected that our results will lead to useful applications such as kinetic analysis of various biomolecules in a single cell and identification of environmental materials in the atmosphere, which are unknown due to their significantly small amount, among others. References and Notes (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) Futamata, M.; Bruckbauer, A. Jpn. J. Appl. Phys. 2001, 40, 4423. (4) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58 (11), 324A–338A. (5) Gray, S. K. Plasmonics 2007, 2, 143–146. (6) Shimada, T.; Imura, K.; Hossain, M. K.; Okamoto, H.; Kitajima, M. J. Phys. Chem. C 112 (11), 4033-4035. (7) Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. Rapid Commun. Mass Spectrom. 1988, 2, 151. (8) Tanaka, K. BUNSEKI 1996, 4, 253–261. (9) Fenner, N. C.; Daly, N. R. ReV. Sci. Instrum. 1966, 37, 1068– 1070. (10) Vastola, F. J.; Pirone, A. J. AdV. Mass. Spectrom. 1968, 4, 107– 111.
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