Ion-Desorption Efficiency and Internal-Energy Transfer in Surface

Article pubs.acs.org/JPCC

Ion-Desorption Efficiency and Internal-Energy Transfer in SurfaceAssisted Laser Desorption/Ionization: More Implication(s) for the Thermal-Driven and Phase-Transition-Driven Desorption Process Kwan-Ming Ng,*,† Siu-Leung Chau,† Ho-Wai Tang,† Xi-Guang Wei,‡ Kai-Chung Lau,*,‡ Fei Ye,§ and Alan Man-Ching Ng§ †

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China Department of Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, People’s Republic of China § Department of Physics, South University of Science and Technology of China, Shenzhen, Guangdong, People’s Republic of China 518055 ‡

S Supporting Information *

ABSTRACT: Fundamental factors governing the ion-desorption efficiency and extent of internal-energy transfer to a chemical thermometer, benzylpyridinium ion ([BP]+), generated in the surface-assisted laser desorption/ionization (SALDI) process, were systematically investigated using noble metal nanoparticles (NPs), including AuNPs, AgNPs, PdNPs, and PtNPs, as substrates, with an average particle size of 1.7−3.1 nm in diameter. In the correlation of ion-desorption efficiency and internal-energy transfer with physicochemical properties of the NPs, laser-induced heating of the NPs, which are dependent on their photoabsorption efficiencies, was found to be a key factor in governing the ion-desorption efficiency and the extent of internal-energy transfer. This suggested that the thermal-driven desorption played a significant role in the ion-desorption process. In addition, a stronger binding affinity of [BP]+ to the surface of the NPs could hinder its desorption from the NPs, and this could be another factor in determining the ion-desorption efficiency. Moreover, metal NPs with lower melting points could also facilitate the ion-desorption process via the phase-transition process, which could lower the activation barrier (ΔG#) of the iondesorption process by increasing the entropic change (ΔS#). The study reveals that high photoabsorption efficiency, weak binding interaction with analyte molecule, and low melting point could be critical for the design of SALDI substrates with efficient ion desorption.

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INTRODUCTION Surface-assisted laser desorption/ionization (SALDI), a major branch of laser desorption/ionization (LDI) techniques, has been widely applied to mass spectrometry (MS) analysis of small molecules, and has become increasingly popular for analysis of environmental samples, forensic samples, drugs, metabolomics, and proteomics, and for imaging mass spectrometric analysis.1−7 A key to its success is the adoption of effective substrates for the efficient absorption and controllable transfer of laser energy, which enables the efficient desorption/ionization of analyte molecules, without inducing extensive fragmentation and without introducing serious interfering background ions. Although SALDI-MS using carbon particles as the substrate was first developed in 1995,8 the technique became popular after the introduction of nanostructured porous silicon surface as the substrate to attain a high LDI efficiency at low laser fluence.9−11 Since then, different types and forms of inorganic-based nanomaterials, including silicon-based,12−15 carbon-based,1,8,16−21 and metalbased nanomaterials,22−31 have been developed as SALDI substrates, though their analytical performances are varied and are highly dependent on their sizes, shapes, and surface properties. © XXXX American Chemical Society

Fundamental study of the LDI process remains a challenging issue. While matrix-assisted laser desorption/ionization (MALDI), using organic acids as a matrix, has been developed since the 1980s, it took about two decades for its mechanism to become better studied and understood.32−35 SALDI using inorganic nanomaterials as substrates may exhibit different laser desorption/ionization mechanisms from that of the MALDI process.3,36−44 Inorganic substrates possessing giant chemical structure(s) could exhibit distinctive physicochemical properties when reduced from bulk to nanosize. For instance, the enhanced photoabsorption efficiency due to change in band gap for interband transition and/or the appearance of plasmonic resonance, and unique thermal properties because of decreased thermal conductivity and melting point, etc., can make the laser−material interaction become more complicated.5,28,36,37,45−48 These make the development of SALDI substrates mainly a trial-and-error approach. It is well-known that physicochemical properties of most inorganic nanomaterials are dependent on their sizes and/or Received: June 22, 2015 Revised: September 14, 2015

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DOI: 10.1021/acs.jpcc.5b05957 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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morphologies.36,37,47−53 The variation of the physicochemical properties resulting from the different sizes or shape of the nanoparticles would lead to a different extent of laser energy deposition, and subsequent laser-induced heating and/or laserinduced phase transition; thus this affects the resultant desorption/ionization efficiency and internal-energy transfer in the SALDI process.5,38,41,46−48,54−57 A recent study using spherical AuNPs of 10 nm diameter showed a 10 times higher LDI efficiency than their 2 nm diameter counterparts for the detection of a cationized peptide (Substance P).24 In another study, spherical AuNPs (30 nm diameter) showed a 2.8 times lower laser fluence threshold at 355 nm for the LDI of a synthetic polymer (PEG600) than AuNP nanorod (18 nm diameter × 50 nm length).58 While the physicochemical properties of SALDI substrates are well-known to affect the laser desorption/ionization efficiencies, the relative importance of the individual properties remains unclear. A main reason is the difficulty in preparing well-controlled nanomaterial substrates with known physicochemical properties for systematic investigation. Furthermore, experimental determination of the physicochemical properties of nanomaterials could be challenging, and these data might not be readily available for correlation studies. As a result, studies on the fundamental physicochemical factors governing the SALDI process are limited, though some qualitative studies have been reported. Another challenge in studying the fundamentals of the SALDI process is the complexity of the indistinguishable desorption and ionization processes occurring in a nanosecond time frame.59 Herein, an ionic salt, namely benzylpyridinium salt, which has been widely used as a chemical thermometer, is adopted to investigate the laser-induced ion-desorption process. In addition, a series of noble metal nanoparticles, including silver nanoparticles (AgNPs), gold nanoparticles (AuNPs), palladium nanoparticles (PdNPs), and platinum nanoparticles (PtNPs), with well-defined physicochemical properties was adopted as SALDI substrates to investigate the effect of various physicochemical properties of the noble metal NPs on the iondesorption efficiencies and the extent of internal-energy transfer to the ions generated from the laser-induced ion-desorption process. Using the argon ion sputtering method, high purity noble metal nanoparticles with similar particle size and morphology were prepared. The high purity and relative inertness of noble metal nanoparticles without coating any stabilizing ligands could make their physicochemical properties relatively constant for the comparison study. The four noble metals have different orders of melting point (Pt > Pd > Au > Ag),60 thermal conductivity (Ag> Au > Pd > Pt),60 specific heat capacity (Pd > Ag > Pt > Au),60 and UV absorptivity.28 Theoretical calculation based on molecular dynamics was also adopted to determine the melting points of the noble metal NPs.61−64 By correlating different physicochemical properties of nanomaterials with the ion-desorption efficiency and the internal-energy transfer, it is expected that the importance of the physicochemical properties governing the SALDI desorption process could be revealed. This would enhance an understanding of the complex SALDI process as well as provide important insights into the rational design and development of more effective SALDI substrates for improving the analytical performance of SALDI-MS, and for extending the scope of potential applications in the future.

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EXPERIMENTAL AND THEORETICAL METHODS

Materials and Chemicals. HPLC grade methanol was purchased from Lab-Scan (>99.9%, Bangkok, Thailand). Analytical reagent grade acetone was purchased from Anaqua Chemicals Supply (>99%, Houston, TX, USA). The glass coverslips (18 mm × 18 mm, thickness ≤0.2 mm) were purchased from Ted Pella, Inc. (Redding, CA, USA). The glass coverslips were washed with methanol followed by acetone before use. Benzylpyridinium chloride was synthesized by the reaction of pyridine (anhydrous, >99.8%, Sigma-Aldrich) with benzyl chloride (>99%, Sigma-Aldrich). Benzyl chloride was mixed with 3 mL of anhydrous pyridine at a molar ratio of 20:1 (pyridine/benzyl chloride) and was heated at 60 °C in a water bath for 5 h. Excess pyridine was removed by vacuum evaporation. The synthesized product was confirmed by SALDI-MS analysis and was used without further purification. Stock solution (2 × 10−2 M) of the benzylpyridinium (BP) salt was prepared in methanol, and the working solution (8 × 10−5 M) was prepared by serial dilution from the stock. Preparation of Noble Metal Nanoparticles. The noble metal nanoparticles (NPs) were generated by argon ion sputtering using a sputter coater (SCD 005; Bal-Tec AG, Liechtenstein). Ultra high purity argon gas (99.999%) was used as the sputtering gas. High purity gold, silver, platinum, and palladium foils (>99.9%; Ted Pella Inc., Redding, CA, USA) were used as sputtering targets. All noble metal targets were circular in shape with diameter 54 mm and thickness 0.1 mm. The sputtering time for the different metals was optimized to generate NPs with diameters of about 1.7−3.1 nm. The sputtering current and time for the generation of silver nanoparticles (AgNPs), gold nanoparticles (AuNPs), palladium nanoparticles (PdNPs), and platinum nanoparticles (PtNPs) were 4 mA, 4 s; 10 mA, 5 s; 12 mA, 10 s; and 12 mA, 14 s, respectively. The sputtering distance between the glass coverslips and the metal target was 80 mm. The chamber was first evacuated to a pressure of 0.02−0.2 mbar and flushed with argon gas four times before metal sputtering. The chamber pressure was maintained at 0.04−0.05 mbar by adjusting the flow of argon gas. Transmission Electron Microscopic (TEM) Examination. In the examination of the morphology and size of noble metal NPs, the NPs coated on transmission electron microscopy grids (Formvar/Carbon 400 mesh Cu Grid; SPI Supplies, West Chester, PA, USA) were examined under a transmission electron microscope (Tecnai G2 20 S-TWIN; FEI, Hillsboro, OR, USA). The particle size of the four types of noble metal NPs was determined by image analysis of the respective TEM micrographs using ImageJ (version 1.45s, NIH, USA). Three micrographs (covering an area of ∼88 × 88 nm2) were acquired for AuNPs, PtNPs, and PdNPs. For AgNPs, 11 micrographs (covering an area of ∼88 × 88 nm2) were acquired. The number and average size of the nanoparticles in each TEM micrograph were determined using the built-in “Analysis Particles” function of the ImageJ program. Mass Spectrometric Measurement. To prepare samples for the determination of ion-desorption efficiency and internalenergy transfer, the samples of BP salt were prepared on glass coverslips sputtered with noble metal NPs. The NP-coated glass coverslips were adhered onto a MALDI plate using electrically conductive double-sided tape (9713 XYZ-Axis, 3M, St. Paul, MN, USA). To apply samples on the metal NP substrates, 1.0 μL of BP solution was deposited onto sample B


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The Journal of Physical Chemistry C wells of sputtered glass coverslips. The concentration of BP solution used for the analysis was 8 × 10−5 M to obtain sufficient ion intensity without saturating the microchannel plate detector. All mass spectrometric measurements were performed with a Bruker Daltonics Ultraflex II MALDI TOF/TOF (Bremen, Germany) equipped with a 355 nm Nd:YAG solid state smartbeam laser (with pulse duration of 3 ns). The laser pulse energy was calibrated using a laser energy meter (Molectron EM-400, Coherent, Santa Clara, CA, USA) with a pyroelectric sensor (J8LP, Coherent). The mass spectrometer was operated in the positive ion mode and linear mode, with other instrumental parameters stated in the following: ion source 1, 25.16 kV; ion source 2, 23.96 kV; lens voltage at 5.74 kV; pulsed ion extraction at 95 ns; laser pulse intensity in the range 69.8−78.8% of the maximum energy, corresponding to the laser fluence of 15.3−83.7 mJ cm−2; and laser pulse frequency at 6 Hz. The dimension of the laser spot in circular shape was ∼135 μm in diameter. The mass spectra were acquired in the m/z range 20−1500. The instrument was controlled via the Bruker Daltonics Flex control software (version 2.4; Bruker Daltonics GmbsH, Bremen, Germany). The mass spectra data were analyzed using Flex Analysis (version 1.2; Bruker Daltonics GmbsH, Bremen, Germany). Determination of Ion-Desorption Efficiency and Internal-Energy Transfer. In the determination of desorption efficiencies of BP ions using different noble metal NPs as SALDI substrates, each sample spot was evenly rastered with a total of 2202 laser shots to generate a mass spectrum. The total intensity of desorbed BP ions was the summation of the intensity of [BP]+ at m/z 170 and the fragment [BP − pyridine]+ at m/z 91 recorded in the mass spectrum. All the measurements were replicated (n = 4−6) to determine the mean and the standard deviation (SD) of the ion intensities. Since the four noble metal NPs coated on the glass slides have different coverage densities, to have a fair comparison of the ion-desorption efficiencies of the different noble metal NPs, the total intensity of desorbed BP ions was normalized by the total section area of NPs in a circle sample well with a diameter of 3 mm. The details of normalization are described in the Supporting Information. For the determination of the internal energy of ions generated by SALDI using the noble metal NPs as the substrates, the simplified survival yield (SY) method using BP ions as chemical thermometer ions was adopted here. This simplified SY method has been previously adopted for the study of the SALDI process using carbon-based nanomaterials,41 and also used in a MALDI matrix study.65 In short, this method is based on the correlation of the experimentally determined dissociation rate coefficients (kexp) of BP ions to its internalenergy-dependent rate coefficient curve. Extensive details have been previously described41,66−69 and only a brief summary is given here. BP ion, [BP]+, could undergo a simple bond cleavage at the C−N bond (between the benzyl C and pyridine N) when it possesses an amount of internal energy greater than the critical energy of the dissociation reaction (Eact) (scheme I). As a result, a benzyl cation and a neutral pyridine molecule would be formed: [BP]+ → [BP − pyridine]+ + pyridine m /z 170 m /z 91

The extent of fragmentation can be expressed in terms of SY. The SY was defined as the relative proportion of intact [BP]+, which was numerically the fraction of the intensity of [BP]+ (Im/z 170) over the total intensity of [BP]+ and [BP − pyridine]+ (Im/z 170 + Im/z 91). SY = [Im / z 170/(Im / z 170 + Im / z 91)] × 100%

(1)

A lower SY (%) reflects a higher degree of internal-energy transfer, as a smaller proportion of [BP]+ possesses internal energy lower than the Eact for the fragmentation. The SY was used for calculating the ion dissociation rate constant (kexp) according to eq 2, where t0 is the dissociation time frame and it is approximated to be the delay time (t0 = 95 ns) for ion extraction in the ion source: kexp = −(1/t0) ln(SY)

(2)

The internal-energy-dependent rate coefficient curve for [BP]+ was derived using the MassKinetic program.70 The calculation was based on the Rice−Ramsperger−Kassel− Marcus (RRKM) statistical theory.70,71 The critical energy for the fragmentation of [BP]+ was calculated at the B3LYP level of theory, using a 6-31G* basis set.69 The vibrational frequencies of the optimized [BP]+ at the ground state and the transition state used for calculating the density of states, ρ(E), of the reactant, and the number of states of the transition state, G*(E − Eact), were determined at the AM1 level of the theory.41,65,66 By correlating the kexp to the derived internal-energy-dependent rate coefficient curve, as shown in Figure S1 (Supporting Information), the average internal energies of [BP]+ desorbed from the different types of the noble metal NPs were determined. A recent study reported by Barylyuk et al. revealed that additional fragmentation of [BP − pyridine]+ could lead to inaccurate determination of internal energy.72 Here, we found that the additional fragment ions recorded at m/z 65 due to the loss of C2H2 from [BP − pyridine]+ were minor fragment ions and that the abundance of the fragment ions was smaller than 2% of the total ion intensities, thus suggesting that the application of the simplified SY method for the determination of the extent of internal-energy transfer without considering the minor fragment ions was reliable. Determination of the Melting Points of Noble Metal NPs by Molecular Dynamics Simulation. The molecular dynamics (MD) simulations were performed on unsupported spherical AgNPs, AuNPs, PdNPs, and PtNPs using the largescale atomic/molecular massively parallel simulator (LAMMPS).73 The NPs were constructed based on the facecentered-cubic unit cells of Ag, Au, Pd, and Pt metals, and all had a diameter of 2.5 nm. Totals of 459 and 555 atoms were in the AgNPs/AuNPs and PdNPs/PtNPs, respectively. The embedded-atom-method (EAM) potentials74,75 (built-in in the LAMMPS program) were employed to model the PdNPs and PtNPs, while the second-nearest-neighbor modified embedded-atom potentials76 (MEAM-2nn) were used on the AgNPs and AuNPs. The MEAM-2nn potential was developed by Baskes and co-workers to address the structural instability and incorrect surface reconstructions found in the first-nearestneighbor modified EAM potentials. All current calculations used radical cutoff distances of 4.50 Å (AuNPs and AgNPs) and 5.30 Å (PdNPs and PtNPs). The MD simulations were carried out in the equilibrium and production stages. The simulation first ran for 500 ps (1 × 106

(I) C


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Figure 1. Transmission electron microscopic images of (a) AgNPs, (b) AuNPs, (c) PdNPs, and (d) PtNPs generated using argon ion sputtering technique (scale bar 20 nm for the overview, 5 nm for the inset). The sputtering current and time for AgNPs, AuNPs, PdNPs, and PtNPs were 4 mA, 4 s; 10 mA, 5 s; 12 mA, 10 s; and 12 mA, 14 s respectively, while the sputtering pressure (ca. 4 × 10−2 mbar) and distance (80 mm) were kept constant.

795 K (AgNPs), 835 K (AuNPs), 980 K (PdNPs), and 1056 K (PtNPs). The melting temperature of AuNPs (835 K) was consistent with experimental scanning electron diffraction measurements (∼825 K).77 The melting temperature of PdNPs (980 K) was in fair agreement with that of 456-atom PdNPs (1090 K) determined by Bhethanabotla et al.78 using the many body potential function. Kim and Lee79 investigated a larger size of 3 nm PtNPs using the semiempirical thermodynamic model, and their predicted melting temperature (1470 K) was higher than the simulated value (1056 K).

steps and a time step of 0.0005 fs) at 100 K to achieve equilibrium for metal NPs. In the production stage, the simulations ran for another 500 ps (1 × 106 steps and a time step of 0.0005 fs) to obtain statistical data in the canonical NVT ensemble (constant number of atoms N, constant box volume V, and constant system temperature T). The MD simulations were then repeated at temperatures from 100 to ∼1500 K with an interval of 5 K. The potential energy at each production run was then taken out and plotted against the temperature, as shown in Figure 8. The melting temperatures of AuNPs, AgNPs, PdNPs, and PtNPs can be identified by the abrupt change in the potential energy (Figure 8). The atomic structure at different temperatures is also given in Figure S2 (Supporting Information). In general, three stages are found in the potential energy plots. In the first stage, the potential energy increases constantly with temperature. The NPs almost remain in their spherical shape, as shown in Figure S2a−d (Supporting Information). The second stage is represented by the premelting process of NPs shown in Figure S2e−h (Supporting Information). The potential energy curve begins to show an upward curvature prior to the melting transition and is followed by a sudden jump. This stage involves the loss of solid rigidity at the surface and the release of latent heat by NPs. In the last stage, the potential energy again increases steadily with temperature. The NPs almost turn into liquid, as depicted in Figure S2i−l (Supporting Information). In the MD simulations, the melting temperatures of 2.5 nm spherical NPs were determined to be

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RESULTS AND DISCUSSION

Generation of Noble Metal Nanoparticles. Four types of noble metal nanoparticles (NPs), including silver nanoparticles (AgNPs), gold nanoparticles (AuNPs), palladium nanoparticles (PdNPs), and platinum nanoparticles (PtNPs), with similar average particle diameters of 1.7−3.1 nm were generated using the argon ion sputtering technique, as depicted in Figure 1. These metal NPs were pseudospherical, with different coverage densities (AgNPs:AuNPs:PdNPs:PtNPs = 1:15:19.7:20.1), as shown in Table S1 (Supporting Information). As these metal NPs were prepared by a physical method (i.e., ion sputtering) from corresponding pure metal targets, they had a naked surface without stabilizing ligands. In addition, the NPs were individually separated without aggregation. Hence, the intrinsic physicochemical properties of the NPs could be assumed to be the same as those of a standalone pure metal NP, without considering any interference effect of

D


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The Journal of Physical Chemistry C stabilizing ligands or additional effects due to the close contact between neighboring NPs. Here, the argon ion sputtering technique offered an advantage that individually separated metal NPs of similar size and morphology could be readily generated, which allowed the comparative studies on the effect of different physicochemical properties of the metal NPs on the efficiency of laser-induced ion desorption and the extent of internal-energy transfer in the SALDI process. Ion-Desorption Efficiency of Noble Metal NPs. Figure 2 shows the desorption of benzylpyridinium ion ([BP]+ at m/z 170) from AgNPs, AuNPs, PdNPs, and PtNPs upon the irradiation with a Nd:YAG solid-state laser at 355 nm. Moreover, the fragment ion, [BP − pyridine]+ at m/z 91, was also detected from the four metal NPs. In addition, one minor fragment ion at m/z 65 due to the loss of C2H2 from [BP − pyridine]+ was also recorded when AgNPs and AuNPs were used as the substrate, However, the total abundance of the minor fragment ions was below 2% of the total ion intensity. Moreover, the minor fragment ion was not observed when PdNPs and PtNPs were used as the substrates. Metal cluster ions, including Ag+ (m/z 107 and 109) and Ag2+ (m/z 214, 216, 218), were abundantly generated from AgNPs, while for AuNPs, metal cluster ions including Au+ (m/z 197), Au2+ (m/z 394, data not shown), and Au3+ (m/z 591, data not shown) were weakly detected. However, no metal cluster ions were observed for PdNPs and PtNPs. The much higher abundance of Ag cluster ions relative to those of Au could be due to the lower melting point and lower ionization potential of AgNPs (795 K, 7.6 eV) compared to those of AuNPs (835 K, 9.2 eV), as summarized in Table 1; this could ease the sputtering and ionization of the metal clusters upon the laser irradiation. However, for PdNPs (980 K) and PtNPs (1056 K), their higher melting points might explain their less favorable formation of metal cluster ions upon the laser irradiation, despite their lower ionization potentials (PtNPs, 9.0 eV; PdNPs, 8.3 eV) than that of AuNPs (9.2 eV). To investigate the effects of the different noble metal NPs on the ion-desorption efficiencies, the total ion intensities, in terms of the summation of the intensity of [BP]+ and the fragment ion [BP − pyridine]+ measured over a range of laser fluence, were normalized by the total section area of NPs in a MALDI sample well with a diameter of 3 mm, as shown in Figure 3. The normalized total ion intensities for the four noble metal NPs increased with the laser fluence. A similar trend in the iondesorption efficiency was also observed in our previous studies using different carbon allotropes as the SALDI substrates,41 and was consistent with the studies using silicon materials and other metal NPs as substrates.39,40 It had been suggested that the higher laser fluence could enhance the photo energy deposition, thus facilitating the thermal desorption process via laserinduced heating of SALDI substrates.13,23,45,46,54 Moreover, another possible reason could be the enhanced laser-induced phase transition with the increasing laser fluence. In fact, our previous study and several reports from other research groups have also revealed that laser-induced phase transition or surface restructuring/melting of SALDI substrates could facilitate the ion-desorption process.39−41,48,80 It is noted that the four noble metal NPs with similar particle sizes did show different ion-desorption efficiencies (AgNPs > AuNPs > PdNPs > PtNPs). For instance, at the laser fluence of 52.8 mJ cm−2, AgNPs exhibited the highest ion-desorption efficiency, which was about 1.6 and 7 times higher than those of AuNPs and PdNPs respectively, while the ion-desorption

Figure 2. Positive SALDI mass spectra of benzylpyridinium ions ([BP]+ at m/z 170, [BP − pyridine]+ at m/z 91) using (a) AgNPs, (b) AuNPs, (c) PdNPs, and (d) PtNPs as the substrates. Mass spectra (a)−(d) were recorded at the laser fluence of 52.8 mJ cm−2.

efficiency of PtNPs was the lowest among the four types of NPs and was 40 times weaker than that of AgNPs. The different iondesorption efficiencies among the metal NPs could be E


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The Journal of Physical Chemistry C Table 1. Melting Points, Ionization Potentials, and Thermal Conductivities of Ag, Au, Pd, and Pt in Bulk Size and/or Nanosize thermal conductivity (W m−1 K−1)

melting point (K) metal NPs

bulk metalsa

NPsb

IPc (eV)

bulk metalsd

NPse

AgNPs AuNPs PdNPs PtNPs

1235 1337 1828 2041

795 835 980 1056

7.6 9.2 8.3 9.0

429 317 84.7 71.6

27.60 16.35 19.49 16.99

a

The melting points of noble metals in bulk size are taken from ref 60. The melting points of noble metal NPs at a diameter of 2.5 nm were calculated by molecular dynamics (MD) simulations using the largescale atomic/molecular massively parallel simulator (LAMMPS).73 c The ionization potentials (IPs) of noble metals in bulk size are taken from ref 60. dThe thermal conductivities of noble metals in bulk size are taken from ref 60. eThe thermal conductivities of noble metal NPs were calculated based on eq S3 in the Supporting Information. The diameters of AgNPs, AuNPs, PdNPs, and PtNPs were 3.1, 1.9, 1.7, and 1.8 nm, respectively. b

Figure 4. Internal-energy transfer of benzylpyridinium (BP) ions desorbed from the various noble metal NPs over a range of laser fluence.

[BP]+ via the AgNPs increased with the laser fluence in a lower laser fluence range from 22.4 to 43.9 mJ cm−2, while the internal-energy transfer for AuNPs increased from 22.4 to 58.2, thus showing a heating effect, but the extent of internal-energy transfer became relatively constant when the laser fluence increased beyond 43.9 and 58.2 mJ cm−2, respectively. For PdNPs and PtNPs, the internal energy of [BP]+ desorbed was increased over a range of laser fluence from 36.7 to 83.7 mJ cm−2. The results reveal that increasing the laser fluence could increase the ion-desorption efficiency and the extent of internalenergy transfer from the metal NPs to the [BP]+. Moreover, it was noted that the different types of metal NPs could exhibit a significant effect on the extent of internal-energy transfer to [BP]+ (AgNPs > AuNPs > PdNPs > PtNPs) upon laser irradiation at the same laser fluence (Figure 4). For instance, at the laser fluence of 52.8 mJ cm−2, the average internal energy of [BP]+ desorbed from AgNPs was 6.18 ± 0.03 eV, which was higher than that of [BP]+ desorbed from AuNPs (5.65 ± 0.05 eV), followed by PdNPs (5.18 ± 0.02 eV) and PtNPs (4.77 ± 0.17 eV). A similar trend of results was also observed at other laser fluences. The results suggested that some difference(s) in physicochemical properties among the metal NPs could govern the extent of internal-energy transfer, and will be discussed in the following section. In addition, we also found that the iondesorption efficiencies of the different noble metal NPs increased with their corresponding internal-energy transfer, as shown in Figure 5, which implied again that the thermal-driven

Figure 3. Effect of laser fluence on the normalized total ion intensity of benzylpyridinium (BP) ions desorbed from AgNPs, AuNPs, PdNPs, and PtNPs. The one-sided error bars were determined from replicate measurements (n = 4−6).

attributed to their different physicochemical properties and would be discussed subsequently. However, it was surprising to note that a reverse order of the ion-desorption efficiencies, i.e., PtNPs > AuNPs > AgNPs, was reported by Yonezawa and coworkers.28 The discrepancy could be due to the large variation in particle sizes (2−30 nm) among the different NPs adopted in Yonezawa’s study. The NPs prepared by laser ablation in aqueous media would result in variation in sizes and surface areas, and thus might lead to variations in physicochemical properties and the different order of ion-desorption efficiencies. Another possible reason could be due to the involvement of protonation (ionization) process for the detection of peptides adopted in Yonezawa’s study, in which the noble metal NPs might facilitate the ionization process in a way which was different from that of the desorption of the BP ions adopted in the current study. Internal-Energy Transfer via Noble Metal Nanoparticles. The survival yields of [BP]+ desorbed from the AgNPs, AuNPs, PdNPs, and PtNPs over a range of laser fluence are shown in Figure S3 (Supporting Information). The corresponding average internal energies of the [BP] + determined from their extent of fragmentation within a dissociation time frame of 95 ns are depicted in Figure 4. It was noted that the extent of internal-energy transfer to the

Figure 5. Correlation of normalized total ion intensity of benzylpyridinium (BP) ions desorbed from the various noble metal NPs with their corresponding internal-energy transfer at the laser fluence of 52.8 mJ cm−2. F


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Table 2. Specific Heat Capacities, Densities, Dielectric Permittivities (Real and Imaginary Parts, ε′ and ε″), Photoabsorption Efficiencies (Qabs), and Laser-Induced Heating Temperatures of AgNPs, AuNPs, PdNPs, and PtNPs metal NPs

sp heat capacitya (J g−1 K−1)

density at 298 Kb (g cm−3)

ε′c

ε″c

Qabsd

laser-induced heating tempe (K)

AgNPs AuNPs PdNPs PtNPs

0.235 0.129 0.244 0.133

10.5 19.3 12.0 21.5

−2.04 −1.24 −5.54 −3.96

0.28 5.60 6.53 8.18

0.185 25 0.005 51 0.003 65 0.003 53

18 836 943 569 542

a

The specific heat capacities of noble metals are taken from ref 60. bThe densities of noble metals are taken from ref 60. cThe dielectric permittivities (ε′ and ε″) of noble metal NPs were calculated based on Johnson and Christy’s work in ref 81. dThe photoabsorption efficiencies (Qabs) of noble metal NPs at the laser irradiation wavelength of 355 nm were calculated using eq 3, based on the work of Luk’yanchuk et al. in ref 83. eThe temperatures of noble metal NPs upon laser-induced heating at a laser fluence of 52.8 mJ cm−2 were calculated using eqs 4 and 5, based on the work of Luk’yanchuk et al. in ref 83. The diameters of AgNPs, AuNPs, PdNPs, and PtNPs were 3.1, 1.9, 1.7, and 1.8 nm, respectively.

effect of laser-induced heating on the ion-desorption efficiency and the extent of internal-energy transfer, the temperature of the noble metal NPs upon laser irradiation was estimated for the correlation with the ion-desorption efficiency and internalenergy transfer. The photoabsorption efficiency (Qabs) of a metal NP upon laser irradiation at a particular wavelength can be determined from eq 3, based on the Rayleigh approximation, which is valid when the size of the metal NPs (∼1.7−3.1 nm) is much smaller than the wavelength (e.g., 355 nm) of electromagnetic radiation.85,86

desorption might play an important role in the ion-desorption process. For mass spectrometric detection of analytes, it is desirable to desorb and ionize analytes without inducing serious fragmentation. Here, the relatively soft nature of PtNPs among the other NPs would be more favorable for the detection of analytes in an intact form. Physiochemical Properties of Noble Metal Nanoparticles and Their Effects on Ion-Desorption Efficiency and Internal-Energy Transfer. To investigate the fundamental factors governing the laser-induced ion-desorption process, the measured ion-desorption efficiencies and the internal-energy transfer were correlated with various physicochemical properties of the NPs, including thermal conductivities, laser-induced heating temperatures, binding affinities of [BP]+, and melting points of the NPs. Various physicochemical properties of the noble metal in a bulk form and/or in the form of NPs are summarized in Tables 1 and 2. Effect of Thermal Conductivity. Thermal conductivities of SALDI substrates had been reported to be a critical factor affecting the efficiency of the SALDI process.5,37,47,54,80 A substrate with a lower thermal conductivity could induce the thermal confinement effect; i.e., a larger amount of energy would be trapped in the substrate due to the poorer dissipation of thermal energy, thus resulting in the superheating of the NPs. The details of the calculation of thermal conductivities are provided in the Supporting Information. The thermal conductivities of AgNPs, AuNPs, PdNPs, and PtNPs in the size range 1.7−3.1 nm are found to be approximately 4−20 times lower than those of their corresponding bulk forms, as summarized in Table 1, thus suggesting that thermal confinement effect is very likely existent upon the laser irradiation of the NPs. It is believed that a lower thermal conductivity of SALDI substrates could result in the superheating of NPs upon the laser irradiation and thus achieve a higher extent of internalenergy transfer and a higher ion-desorption efficiency. However, the correlation plots of internal-energy transfer and ion-desorption efficiencies with the thermal conductivities as shown in Figure S4 (Supporting Information) did not show an obvious trend, and the results revealed that there may be other physicochemical properties exerting more significant effects on the ion-desorption efficiency and the internal-energy transfer, thus masking the effect of thermal conductivities of the NPs. Effect of Laser-Induced Heating. Photoabsorption via electronic interband and/or intraband transitions is a key process for the energy deposition in the metal NPs upon laser irradiation,37,81−84 which could subsequently promote the laserinduced heating and phase transition process of the NPs and also induce desorption of absorbed analytes. To investigate the

Q abs = q

12ε″ (ε′ + 2)2 + ε″2

(3)

where ε′ and ε″ are the respective real part and imaginary part of the dielectric permittivity (ε) of the metal,81,87 and are closely related to the electronic interband structure and the availability of localized surface plasmon, respectively;81,88 q = R/λ, where R is the radius of metal NPs and 1/λ is the wavenumber of electromagnetic radiation. The Qabs values of the four NPs over a range of wavelength are shown in Figure S5 (Supporting Information). Assuming that heat loss from a NP is negligible (i.e., a thermally isolated NP), the conversion of a specified laser fluence (Φ, in J cm−2) into the density of internal absorbed energy (E, in J cm−3) of the NP can be estimated by the linear relation described by eq 4,83,89 and the maximum temperature that the NP could achieve at the specified laser fluence can be determined by eq 5. E=

3 Q abs Φ 4 R

E = CρT

(4) (5)

C is the specific heat capacity of metal materials, ρ is the density of the materials, and T is the laser-induced heating temperature of the NPs. The temperatures of the metal NPs, including AgNPs, AuNPs, PdNPs, and PtNPs, upon laser irradiation (355 nm) at a laser fluence of 52.8 mJ cm−2, were determined and are summarized in Table 2. The laser-induced heating temperature of the metal NPs upon irradiation at a fixed laser fluence was mainly governed by the photoabsorption efficiencies (Qabs) of the metal NPs (i.e., Qabs at 355 nm, AgNPs > AuNPs > PdNPs > PtNPs, as shown in Table 2). The higher the Qabs value is, the higher is the laser-induced heating temperature of the metal NPs. However, the theoretically calculated laser-induced heating temperature of AgNPs (18 836 K) based on the assumption of negligibility of heat loss (i.e., thermally isolated NP) was not valid, as the formation of abundant Ag cluster ions (i.e., Ag+, Ag2+, Ag3+, etc.) upon laser G


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believed that the effect of particle neighboring distance may be less significant for laser-induced heating occurring in a vacuum compared to the dense environment of solution. Effect of Binding Affinities of [BP]+ to Noble Metal NPs. It is anticipated that the stronger binding interaction of [BP]+ to the surface of the noble metal NPs could hinder the desorption of [BP]+, thus decreasing the ion-desorption efficiency. Here, with the assumption that the total intensity of [BP]+ is proportional to the ion-desorption rate, and the desorption rate is mainly governed by the thermal energy (temperature) of the metal NPs, the binding affinities of the [BP]+ to the surface of the metal NPs could be estimated using the Arrhenius equation, as shown in eq 6.

irradiation could dissipate a significant portion of deposited laser energy, as shown in Figure 2a. In contrast, the assumption was more applicable in the cases of AuNPs, PdNPs, and PtNPs, as negligible or no metal cluster ions were generated upon laser irradiation, and the theoretically calculated laser-induced heating temperatures of AuNPs, PdNPs, and PtNPs fell in a realistic range of 542−943 K and thus were used for the correlation study. Figure 6 shows the correlation of the laser-induced heating temperatures of AuNPs, PdNPs, and PtNPs with their

I[BP]+ = Ae−Ea / RT

(6)

I[BP]+ is the total intensity of [BP]+, Ea is the binding energy, R is the gas constant, T is the laser-induced heating temperature of the metal NPs, and A is the pre-exponential factor. By fitting the total intensity of [BP]+ with the temperature of the metal NPs, as shown in Figure 7, the binding affinities of [BP]+ to the surface of the AuNPs, PdNPs, and PtNPs were determined to be 21.9, 26.0, and 31.3 kJ mol−1, respectively. In addition to the effect of the laser-induced heating temperature, the higher binding energies of [BP]+ to the surface of the metal NPs (AuNPs < PdNPs < PtNPs) could be another factor that partly accounted for the order of the corresponding ion-desorption efficiencies (AuNPs > PdNPs > PtNPs). On the other hand, it was worth noting that the plot of ln I[BP]+ against 1/T for the AuNPs (Figure 7a) showed a tendency of leveling off when the laser-induced heating temperature of the AuNPs increased beyond 1039 K, which might result in the phase transition (or melting) process of the AuNPs when the laser-induced heating temperature was beyond its melting point. Indeed, at the point of leveling off, the laser-induced heating temperature of AuNPs was about 200 K higher than the theoretically calculated melting point of AuNPs (835 K). Moreover, the phase transition might also explain the leveling off of the internalenergy transfer for AuNPs (Figure 4) when the laser fluence was beyond 58.2 mJ cm−2 (which corresponds to the laserinduced temperature of 1039 K). In contrast, in the plots for PdNPs and PtNPs (Figure 7b,c), the ln I[BP]+ kept increasing in a linear manner over the temperature ranges of 396−902 and 451−859 K, respectively, in which the highest temperatures were below the theoretically calculated melting points of PdNPs (980 K) and PtNPs (1056 K), and thus the internalenergy transfer for the PdNPs and PtNP kept increasing over the range of applied laser fluence (Figure 4). It is anticipated that the surface of the metal NPs may not be intact upon laser irradiation (as reflected from the detection of metal cluster ions, such as Au+, Au2+, and Au3+ for the AuNPs) and it might not be ideal to use the Arrhenius equation for estimating the binding affinities of the BP ions to the surface of the metal NPs. However, the linear manner of the Arrhenius plots, as shown in Figure 7, revealed that the extent of the surface change of the metal NPs might not be significant to affect the estimation of the binding affinities. Effect of Melting Point. It has been suggested that the lower melting points of SALDI substrates could enhance the iondesorption efficiency via the phase transition process of SALDI substrates.39−41,48,80 It is well-known that melting points of metals can be significantly lowered when they are reduced from bulk form to NPs, which is due to the extremely large surface-

Figure 6. Correlation of (a) normalized total ion intensity and (b) internal-energy transfer of benzylpyridinium (BP) ions desorbed from the various noble metal NPs at the laser fluence of 52.8 mJ cm−2 with their corresponding temperatures.

respective ion-desorption efficiencies and internal-energy transfer. The results show that higher temperatures of the metal NPs could enhance both the ion-desorption efficiencies and the extent of internal-energy transfer to the [BP]+, thus revealing that a thermal-driven desorption process played a significant role in the ion-desorption process. In addition to physicochemical properties of NPs, a previous study suggested that a closer particle neighboring distance would result in a higher temperature increase for NPs upon laser irradiation in solutions.90 As the AgNPs were relatively separated further away than the other NPs, we reduced the particle neighboring distance of AgNPs by >50% (from 18.4 to 7.5 and 6.9 nm) by decreasing the sputtering distance (from 80 to 70 and 60 mm, respectively), as summarized in Table S2 and Figure S6 (Supporting Information). However, no significant change in the extent of internal-energy transfer and iondesorption efficiency were observed (Figure S7, Supporting Information). Further decreasing the sputtering distance to 50 mm for reducing the particle neighboring distance caused the aggregation of AgNPs, which significantly increased the particle size to 4.9 nm, and thus was not used for comparison. We H


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Figure 7. Correlation of natural logarithm of normalized total ion intensity of benzylpyridinium (BP) ions (ln I[BP]+) desorbed from (a) AuNPs, (b) PdNPs, and (c) PtNPs with the reciprocal of corresponding temperature (1/T). The binding energy (Ea) was calculated according to the derivation of the Arrhenius equation (ln I[BP]+ = −Ea/RT + ln A), i.e., Ea = slope × R. Figure 8. Potential energies as a function of temperature for (a) AgNPs, (b) AuNPs, (c) PdNPs, and (d) PtNPs. The melting temperatures of the noble metal NPs are identified by the abrupt change in the potential energy (indicated by arrows).

to-volume ratio of NPs resulting in the much higher free energy of the NPs. The potential energies as a function of temperature for the noble metal NPs and their respective melting temperatures are shown in Figure 8. By adopting the molecular dynamics simulation method, the melting points of PdNPs (555 atoms), PtNPs (555 atoms), AuNPs (459 atoms), and AgNPs (459 atoms) with particle size of 2.5 nm were determined and are summarized in Table 1. Here, at a fixed laser fluence of 52.8 mJ cm−2, we found that the ion-desorption efficiencies of the noble metal NPs (AgNPs > AuNPs > PdNPs > PtNPs) were inversely correlated to their melting points (PtNPs at 1056 K > PdNPs at 980 K > AuNPs at 835 K > AgNPs at 795 K), as illustrated in Figure 9. AuNPs and AgNPs, having lower melting points, were found to give higher total ion intensities of [BP]+, while PdNPs and PtNPs with higher

melting points showed lower total ion intensities. The similar trend of results was also observed at other laser fluences (data not shown). This correlation illustrated that the melting point of SALDI substrates could also be an important factor governing the ion-desorption efficiency. The lower the melting point is, the higher the ion-desorption efficiency is. In fact, the phenomenon could be rationalized from the thermodynamics point of view, considering the free energy change of the desorption of [BP]+ from the metal NPs involving the phase transition: ΔG # = ΔH # − T ΔS # I


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ΔG#) of the ion-desorption process by increasing the entropic change (ΔS#). Using the noble metal NPs substrate system, we attempted to untangle the interplay of different physicochemical properties of the substrate governing the ion-desorption efficiency and internal energy transfer, for providing directions for SALDI substrate design. We anticipated that the development of SALDI substrates with high photoabsorption efficiency, weak binding interaction with analyte molecules, and low melting point, would be a rational direction for enhancing the detection sensitivity of SALDI-MS, and our research work making use of alloy nanoparticles (nanoalloy) for tuning the physicochemical properties of SALDI substrates is currently in progress.

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Figure 9. Correlation of normalized total ion intensity of benzylpyridinium (BP) ions desorbed from the various noble metal NPs at the laser fluence of 52.8 mJ cm−2 with their corresponding melting points.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05957. Additional details regarding particle analysis, normalization of total ion intensity, and calculation of thermal conductivities; figures regarding the internal-energydependent rate coefficient (K (E) ) curve for the dissociation of benzylpyridinium (BP) ion, atomic structure of the NPs during melting, survival yield of BP ion, correlation with thermal conductivities, photoabsorption efficiency of noble metal NPs, effect of particle neighboring distance, and schematic energy diagram showing the effect of phase-transition process (PDF)

where ΔG# is the activation barrier (i.e., change in Gibbs free energy at the transition state) of the ion-desorption process. ΔH# and ΔS# are the respective enthalpy change and entropy change at the transition state of the ion-desorption process, and T is the temperature of the NPs. The involvement of the phase transition process of the NPs upon the ion-desorption process could increase the entropic change (ΔS#), and thus lower the activation barrier (i.e., ΔG#) for ion desorption, as depicted in Figure S8 (Supporting Information). As a result, a SALDI substrate with a lower melting point could ease the phase transition upon laser irradiation, thus favoring the iondesorption process and achieving higher detection sensitivity.

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AUTHOR INFORMATION

Corresponding Authors

CONCLUSIONS Noble metal NPs, including AgNPs, AuNPs, PdNPs, and PtNPs, with well-controlled particle sizes of 1.7−3.1 nm, and a chemical thermometer, benzylpyridinium ion [BP]+, were adopted for a systematic study on the effect of various physicochemical properties of SALDI substrates on iondesorption efficiency and internal-energy transfer in the SALDI process. Different noble metal NPs exhibited significant effect(s) on ion-desorption efficiency and internal-energy transfer. The effects were attributed to their different physicochemical properties, including photoabsorption efficiencies, melting points of the metal NPs, and the binding affinities of [BP]+ to the surface of the metal NPs. It was found that the amount of thermal energy deposited on noble metal NPs was a key factor governing the ion-desorption efficiency. Also, the photoabsorption efficiency (Qabs) was a key parameter affecting the temperature of metal NPs upon laser irradiation. Together, increasing the temperature of the metal NPs could enhance ion-desorption efficiency and the extent of internal-energy transfer, which reaffirmed the findings of previous studies. Moreover, our results also showed that the binding affinity of [BP]+ to the surface of noble metal NPs could also be a factor affecting the ion-desorption efficiency. The stronger binding strength could hinder the ion-desorption process, and thus a lower ion-desorption efficiency would result. Furthermore, the melting point of the metal NPs could also be an important factor affecting ion-desorption efficiency. Metal NPs having lower melting points could assist the iondesorption process via the phase transition of the metal NPs upon laser irradiation. We proposed that the involvement of the phase transition process could lower the activation barrier (i.e.,

*E-mail: [email protected]. Tel.: +(852)-2219 4696. (K.M.N.). *E-mail: [email protected]. Tel.: +(852)-3442-6849. (K.C.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Frankie Y. F. Chan of the Electron Microscope Unit of The University of Hong Kong for technical assistance in the size measurements of the noble metal nanoparticles using transmission electron microscopy. K.-M.N. acknowledges the funding support of the General Research Fund (Grants HKU_704511P and 17304014) of the Hong Kong Research Grants Council. K.-C.L. and X.-G.W. are grateful for the useful help provided by B.-J. Lee in adapting MEAM-2nn potentials of Au and Ag metals in the LAMMPS program. K.-M.N. thanks Bobby Man and Koey Ma for their preliminary work.

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