Nanosecond UV Laser Ablation of Gold Nanoparticles: Enhancement

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Nanosecond UV Laser Ablation of Gold Nanoparticles: Enhancement of Ion Desorption by Thermal-Driven Desorption, Vaporization or Phase Explosion Samuel Kin-Man Lai, Ho-Wai Tang, Kai-Chung Lau, and Kwan-Ming Ng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06261 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 21, 2016

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The Journal of Physical Chemistry

Nanosecond UV Laser Ablation of Gold Nanoparticles: Enhancement of Ion Desorption by Thermal-Driven Desorption, Vaporization or Phase Explosion †

†

‡

Samuel Kin-Man Lai , Ho-Wai Tang , Kai-Chung Lau , Kwan-Ming 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

* To whom correspondence should be addressed: KM Ng, email: [email protected]

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ABSTRACT Phase transition of SALDI substrates has been identified as a driving process for ion desorption in many previous SALDI fundamental studies. Here, the effect of various phase transition stages, including substrate melting, vaporization, and phase explosion, on SALDI ion desorption efficiency and extent of heat transfer were investigated. We employed molecular dynamics to simulate the phase transition (from melting, vaporization, to phase explosion) of gold nanoparticles (AuNPs, ⌀: 2.5 nm) upon laserinduced heating, and experimentally probed the corresponding SALDI ion desorption efficiency and extent of heat transfer to a chemical thermometer, benzylpyridinium (BP) salt (using 355 nm solid state laser, pulse width: 6 ns, laser fluence range: 21.3 mJ/cm2 to 125.9 mJ/cm2). The results showed that substrate phase explosion has the most significant effect on enhancing the ion desorption efficiency and lowering the extent of heat transfer, which were reflected by an abrupt increase in both the ion desorption efficiency and the survival yield, when the laser fluence exceeded the AuNPs’ phase explosion threshold temperature (5,800K). Compared with phase explosion, vaporization only exhibited a limited effect on the ion desorption efficiency, while the effect of melting was not noticeable and even overridden by the thermal-driven desorption. The significant effect of phase explosion on enhancing the ion desorption efficiency could be attributed to the weaker binding interaction between the BP ions and the Au atoms which were rapidly ablated during the phase explosion stage, and the cooling effect on the BP ions could be due to the adiabatic expansion of the ablation plume during the phase explosion. The study revealed that SALDI substrate with a lower phase explosion threshold would have a higher potential in enhancing the analytical performance of SALDI-MS.

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INTRODUCTION Pulsed laser ablation has been widely applied in different aspects of application, ranging 1-2 from controlled removal of organ tissues for medical surgery to microfabrication of 3-4

nanostructures for material sciences. Two important analytical applications of pulsed laser are matrix-assisted (MALDI) and surface-assisted (SALDI) laser desorption/ionization respectively, which make use of organic compounds and inorganic materials (e.g. Si, C, noble metal) as the laser energy modulator, to desorb/ionize samples 5-19 almost instantly from solid phase to gas phase for mass spectrometric (MS) analysis. In MALDI, the fate of the organic matrix upon pulsed laser irradiation, including a series of physical and chemical processes taken place upon absorption of laser, have been 20-23 studied quite extensively by various experimental and computational techniques. In general, the desorption process of MALDI was believed to be due to the phase explosion of organic matrices which entrained the cocrystallized analyte molecules to the gas 20, 24 phase. These fundamental studies facilitated the development of MALDI-MS in various aspects, such as choice of matrices, choice of laser wavelength and analyte-to25-27 matrix ratio, for the optimization of its analytical performance. Although SALDI-MS is getting more popular, fundamental studies on SALDI process 16, 28-34 remains relatively limited when comparing with the studies on its analytical 35-36

applications. A variety of silicon-based, carbon-based nanomaterials and metallic nanoparticles (NPs), have been reported as efficient SALDI substrates for LDI-MS analysis of small molecules, which complements the limitations of MALDI-MS for the 7, 9-18, 37 low mass analyte analysis ( 99.9%, Bangkok, Thailand). Acetone (analytical reagent grade) was purchased from Anaqua Chemicals Supply (> 99%, Houston, USA). Glass coverslips (18 mm x 18 mm, thickness, ~ 0.16 mm) bought from Ted Pella, Inc. (Redding, CA, USA) were cleaned with methanol followed by acetone before use. Benzylpyridinium (BP) chloride was prepared using pyridine (anhydrous, > 99.8%, Sigma-Aldrich) and benzyl chloride (> 99%, Sigma-Aldrich). The detailed 40 synthesis procedure was described in our previous study. The working solution (6 × 10-5 M) of benzylpyridinium salt (BP) in this study was obtained by successive dilution of stock solution (2 × 10-2 M) using methanol. Preparation of Gold Nanoparticles and Transmission Electron Microscopic Examination. Gold nanoparticles (AuNPs) with an average diameter of 2.5 nm were prepared on glass coverslips by argon ion sputtering with a sputter coater (SCD 005; Bal-Tec AG, Liechtenstein) using high purity gold foil (> 99.9%; Ted Pella Inc., Redding, CA, USA) as sputtering target. The gold target was in circular shape with a diameter of 54 mm and a thickness of 0.1 mm. The sputtering conditions and procedures 40 were described in detail in our previous study. The morphology and size of AuNPs coated on transmission electron microscopy grids (Formvar/Carbon 400mesh Cu Grid; SPI Supplies, West Chester, PA, USA) were 5



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examined using transmission electron microscope (Tecnai G2 20 S-TWIN; FEI, Hillsboro, OR, USA). The number and average size of AuNPs used were analysed and determined from their TEM micrographs (covering an area of ~ 88 × 88 nm2) using the built-in “Analysis Particles” function of ImageJ (version 1.45s, NIH, USA) and summarized in Figure S1 and Table S1 (Supporting Information). Mass Spectrometric Measurement. BP chloride salt was used as the analyte to determine the ion-desorption efficiency and the extent of heat transfer of the AuNPs substrates during the laser desorption/ionization process. AuNPs-coated glass coverslips were stuck on a MALDI plate using electrically conductive double-sided tape (9713 XYZ-Axis, 3M, St. Paul, MN, USA). 1.0 μL BP solution (6 × 10-5 M) was then applied onto each sample well of the sputtered glass coverslips. All mass spectrometric measurements were performed with a Bruker Daltonics Ultraflex II MALDI TOF/TOF (Bremen, Germany) mass spectrometer in positive ion and linear mode. Laser fluence was calibrated using a laser energy meter (Molectron EM-400, Coherent, Santa Clara, CA) with a pyroelectric sensor (J8LP, Coherent). AuNPs-coated glass coverslips on the MALDI plate were then irradiated with a 355 nm Nd:YAG solid state smartbeam laser at 6 Hz (with pulse duration of 6 ns). The voltage of ion source 1 and 2 were set at 25.16 kV and 23.96 kV respectively while the lens voltage was set at 5.74 kV. A pulsed ion extraction delay of 60 ns was used. 70.4 to 100% of the maximum laser energy giving a laser fluence range of 21.3 – 125.9 mJ/cm2 was adopted in the current study. The laser spot was found to be nearly circular in shape with diameter of ~135 μm. All mass spectra were recorded in the m/z range of 20 – 1500. Instrument control was performed via the Bruker Daltonics Flex control software (version 2.4; Bruker Daltonics GmbsH, Bremen, Germany), and mass spectra data were analyzed using Flex Analysis (version 1.2; Bruker Daltonics GmbsH, Bremen, Germany). Determination of Ion-Desorption Efficiency and Heat Transfer. To determine the ion-desorption efficiency of the AuNPs for BP ions, each position in the sample well was irradiated with 6 pulses of the laser, and the total ion signals collected from 367 positions across each sample well were combined to generate a mass spectrum. The summation of BP ions, [BP] +, detected at m/z 170 and its fragment ions, [BP – Pyridine] +, detected at m/z 91, was determined to be the total intensity of BP ions desorbed from the AuNPs. All measured ion intensities were normalized with the coverage area of NPs in a circle sample well with a diameter of 3 mm. The details of normalization are described in the Supporting Information. 6


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Besides acting as an analyte, BP ions was also used as a chemical thermometer to probe the extent of heat transfer from the AuNPs to the BP ions during the laser desorption process. The normalized intensities of [BP]+ and [BP – Pyridine] + detected were used to determine the survival yield (SY) of [BP] + ions in the whole experimental laser fluence range. The SY method had been previously adopted for studying the extent of heat transfer in SALDI process with carbon-based 29 10, 43 40 nanomaterials, silicon-based nanomaterials and noble metal nanoparticles as SALDI substrates. The SY method could determine the extent of heat transfer based on the extent of the fragmentation of BP ions. [BP]+ could dissociate into a benzyl cation ([BP – Pyridine]+) and a neutral pyridine molecule when the heat transfer to the BP ions exceeds the critical energy of the dissociation reaction (Eact) (Scheme I). →

[BP]+

[BP – Pyridine]+ + Pyridine

m/z 170

Scheme I

m/z 91

The extent of fragmentation can thus be expressed in terms of SY, which was defined as the relative proportion of detected [BP]+ to the total intensity of BP ion desorbed (Equation 1): SY = �

𝐼𝑚⁄𝑧170

�𝐼𝑚⁄𝑧 170 + 𝐼𝑚⁄𝑧 91 �

� × 100%

𝑬𝑬 [𝟏]

where 𝐼𝑚/𝑧 170 is the intensity of parent [BP]+ detected, and 𝐼𝑚⁄𝑧 91 is the intensity of fragment ion, [BP – Pyridine] +. The total intensity of BP ion desorbed from the AuNPs is the summation of the intensity of BP ions detected and the intensity of fragment ions. Therefore, the calculated SY is inversely proportional to the extent of heat transfer from the AuNPs to BP ions. In the determination of the iondesorption efficiency and survival yield, replicate measurements (with n = 6 – 8) were performed to determine the mean values and standard deviations (SD) at each laser fluence. Molecular Dynamics Simulation of Laser Substrate Interaction. Large-scale atomic/molecular massively parallel simulator (LAMMPS) was used to perform molecular dynamics (MD) simulations on unsupported spherical AuNP irradiated with a pulse of UV laser. In this work, the computational domain has a length of 4.80 nm in both x and y directions while z direction has a size of 28.68 nm. In the z direction, the AuNP was placed 1 nm away from the bottom of the simulation cell. 7



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Periodic boundary conditions are applied in lateral directions. The AuNP (487 atoms) with diameter of 2.5 nm used in the simulation was constructed based on the face-centred-cubic (fcc) unit cells with lattice constant of 4.08Å. Secondnearest-neighbour modified embedded-atom potentials (MEAM-2nn) was employed for modelling since it predicts various physical properties of both bodycantered-cubic (bcc) and fcc metals, including melting point, specific heat and 59 surface energy, with satisfactory accuracy. The radial cutoff distance was set to 4.5 Å. The initial system was first equilibrated at 298 K for 500 ps (1×107 steps at a time step of 0.05 fs). Laser heating stage was then performed after the equilibrium stage. In order to allow expansion of the ablation plume, the boundary at z direction of the simulation cell was unfixed during laser heating stage. A nanosecond pulsed laser with pulse duration of 6 ns was applied on the AuNP. Here, the laser was described as an energy source term with separated temporal and spatial part in the simulation, as shown as equation 2. 𝐼 = 𝐼𝑜 𝑓(𝑡)𝑔(𝑥, 𝑦, 𝑧)

𝑬𝑬 [𝟐]

where I is the laser beam intensity at position with coordinate (x,y,z) and simulation time t, Io is the total laser beam intensity of the whole laser spot at the 6 ns pulse duration. 𝑓(𝑡) is a Gaussian temporal profile with a 1.5 ns FWHM centered at 3 ns (FWHM𝑡𝑡𝑡𝑡 ), as shown in equation 3. 𝑓(𝑡) =

1

𝜏√2𝜋

exp �−

(𝑡 − 𝑡𝑜 )2 � 2𝜏 2

𝑬𝑬 [𝟑]

where to is the center of the Gaussian peak (3 ns) and τ = FWHM𝑡𝑖𝑖𝑖 /(2√2𝑙𝑙2).

𝑔(𝑥, 𝑦, 𝑧) represents a spatial laser intensity distribution at the irradiated surface (𝑥, 𝑦) and irradiated depth, z. Since 355nm Nd:YAG smartbeam laser used in the experiment has Gaussian shaped envelops with 60µm FWHM diameters in both x and y direction (FWHM𝑠𝑠𝑠𝑠𝑠𝑠𝑠 ), a two dimension Gaussian function was used to 50, 60-63

describe the surface laser intensity distribution. The absorption of laser beam energy of the AuNP was assumed to follow Beer-Lambert law in z direction, meaning that the energy absorbed by the atoms of AuNP drops exponentially from 50, 60-61, 63-65 66 the surface. The spatial function was expressed as equation 4.

8


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(𝑥 − 𝑥𝑜 )2 + (𝑦 − 𝑦𝑜 )2 1 𝑔(𝑥, 𝑦, 𝑧) = exp �− � �� × exp(−𝛼𝛼) 2𝜋𝜎 2 2𝜎 2

𝑬𝑬 [𝟒]

where x, y and z are the coordinates of atoms of AuNPs in the simulation box, 𝛼 is 67 the absorption coefficient of Au metal at wavelength of 355nm (6.21×105 cm-1) and 𝜎 = FWHM𝑠𝑠𝑠𝑠𝑠𝑠𝑠 /(2√2𝑙𝑙2).

To account for the heat loss of the AuNP via the glass coverslip during laser irradiation, a heat dissipation function was also applied on the laser irradiated AuNP in the simulation, as shown in equation 5. Q=

𝑘𝑘 (𝑇 − 298𝐾) 𝑙

𝑬𝑬 [𝟓]

where Q is the resultant heat loss per fs, 𝑘 is the thermal conductivity of silica 68

glass at 300 K (1.36×10-15 Jfs-1m-1K-1) , 𝐴 is the contact area between AuNP and glass coverslip, which is assumed to be the cross-sectional area of the AuNP, 𝑙 is the thickness of the glass coverslip (0.16 mm) and 𝑇 is the instant laser-induced heating temperature during laser irradiation. The laser-induced heating temperature of the substrate is calculated statically using the average kinetic energy of Au atoms in the corresponding simulation cell, which can be described by equation 6. 𝑁

2 ��⃗) 𝑚𝑖 (𝑣 𝚤 𝑇=� 3𝑘𝐵 𝑁 𝑖=1

𝑬𝑬 [𝟔]

where T is the laser-induced heating temperature, 𝑚𝑖 and ��⃗ 𝑣𝚤 are mass and thermal velocity of atom i, 𝑁 is the total number of atoms in a simulation cell and 𝑘𝐵 is the Boltzmann constant (1.38×10−23 JK-1). A number of previous works have suggested the irradiated target had to be separated into two subsystems, electronic subsystems and atomic subsystems, when the thermalization time of laser energy in conduction electron (in fs scale) and the electron-phonon relaxation time (in ps scale) is comparable to the laser duration. A hybrid atomistic-continuum model called two-temperature model was coupled with molecular dynamics (TTM-MD) to describe these processes and to 50, 62, 65, 69-71 calculate the lattice and electron temperatures separately. However, the simulation in this work is purely based on MD since the pulse durations (6 ns) 9



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is much longer than both electron thermalization time (in fs scale) and electronphonon coupling time (in ps scale). Therefore, electron thermalization and electron-phonon coupling are assumed to be completed during simulation and thus both electron and lattice temperatures are equal to the calculated laser-induced heating temperature. The laser heating process was run for 6 ns (1.2×108 steps and a time step of 0.05 fs) and the potential energy of the system was extracted and plotted against laserinduced heating temperature, as shown in Figure 3a. The melting point and boiling point of the AuNP were identified by sudden changes in the slope of the plot. It was found that the melting temperature and boiling temperature of AuNPs are 835 K and 2900 K, respectively. The melting temperature is in well agreement 72 with experimental scanning electron diffraction measurements (~825 K) and our 40

The boiling point is about 200 K lower than that of previous study (~835 K). bulk gold (3129 K), this could be explained by the effect of size reduction to the phase change temperature when the size of material changes from bulk to 73-75 nanoscale. It has been reported that the laser ablation mechanism of a metal could be reflected 50, 64 by the ablation plume content. When the ejection of metal atoms is mainly driven by phase explosion instead of vaporization, the number of metal atom ablated in the ablation plume could increase rapidly. Here, we determined the phase explosion threshold of the AuNP by extracting the number of individual atom ejected to the gas phase from the simulation data as a function of laserinduced heating temperature (Figure 3b). In this study, the phase explosion threshold at ~5800 K was determined from the sudden change of the slope of the plot. In addition, to further study the laser-induced phase change of the AuNP, the evolution of its lattice structure was investigated in detail by plotting the spatial distribution of number density of atoms, ρ𝑁 , at different laser-induced heating temperature, as shown in Figure 4. This method has been used for studying the phase change of various materials, including argon, nickel and silicon, irradiated by pulsed laser and was found useful in describing detailed structural changes 51, 60-62 during short-pulse laser heating. To investigate the effect of different stages of phase change/transition on the mass spectrometric measured ion-desorption efficiency and heat transfer during the laser desorption/ionization process determined in a range of laser fluence, the MD simulation of the AuNP was performed at 18 separate laser fluences which were evenly distributed in the experimental laser range from 21.3 to 125.9 mJcm-2. The 10


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potential energies were plotted as a function of temperature for all the laser fluence. (Figure S2, Supporting Information) In addition, the maximum laser-induced heating temperature for each laser fluence was also calculated. The maximum laser-induced heating temperature which is close to the final laser-induced heating temperature was obtained by taking statistical average of system temperature at the last 0.1 ns of the laser irradiation (2×106 steps at a time step of 0.05 fs). The relationship between the maximum laser-induced heating temperature and the laser fluence is shown in Figure S3 (Supporting Information).

RESULTS AND DISCUSSION Laser Desorption/ Ionization of AuNPs. Upon the UV laser (at 355 nm with 6 ns duration) irradiation of AuNPs coated with benzylpyridinium (BP) salt, abundant BP ions, [BP]+, at m/z 170, were generated, as shown in Figure 1. In addition, the BP ions underwent fragmentation to yield [BP – Pyridine]+, detected at m/z 91. Moreover, gold cluster ions including Au+ (m/z 197), Au2+ (m/z 394), and Au3+ (m/z 591) were also generated in low abundance. It was observed that the intensity of the three Au cluster ions decreased with increasing cluster size (IAu+ > IAu2+ > IAu3+). This could be due to the decrease in binding energy between gold atoms with increase in cluster size, as reported 76 in previous molecular-dynamics simulation studies. Besides gold cluster ions, argon ions (Ar+ at m/z 40) and AuAr+ ions (m/z 237) were also generated upon the laser irradiation of AuNPs. We anticipated that argon gas was trapped in the AuNPs during argon ion sputtering process. When the AuNPs was irradiated with the laser, Ar atom released from the AuNPs would form adduct with Au+ to form AuAr+ ions, and the photo-excited AuAr+ ions would further undergo charge-transfer reaction to generate Ar+ ions (Reaction [1]). AuAr + → Ar + + Au

Reaction [1]

Phase Transition and Heat Transfer of AuNPs during Laser Irradiation. The effect of laser fluence on the total intensity of BP ions and its fragment ions, [BP – Pyridine]+, is depicted in Figure 2a. The total ion intensity showed a gentle increase when the laser fluence increased from 21.3 mJ/cm2 to 74.4 mJ/cm2. The steady increase of the total ion 40 intensity below 80 mJ/cm2 agreed with our previous study at low laser fluence range. The trend could be due to that increasing the laser fluence could increase the amount of photo-energy deposited in the AuNPs and, thus facilitating laser-induced heating of AuNPs, leading to thermal desorption of BP ions. 11



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It is interesting to note that a sudden jump of the total ion intensity was observed when the laser fluence was increased from 74.4 mJ/cm2 to 82.8 mJ/cm2, and the total intensity was increased rapidly by nearly 9 times when the laser fluence was increased from 74.4 mJ/cm2 to 125.9 mJ/cm2, as shown in Figure 2a. We believed that when the laser fluence was beyond a threshold value (e.g., ~ 74.4 mJ/cm2), a dramatic change of the phase, like phase explosion, of the overheated AuNPs might be triggered, and this process might facilitate the desorption of BP ions significantly. In fact, the similar trend of the sudden jump in ion signal could also be observed in the plot of argon ion (Ar+) intensity against the laser fluence, as shown in Figure 2b. We believed that the abrupt increase in Ar+ intensity might also be attributed to the explosive destruction of the AuNPs near the threshold laser fluence (e.g., ~ 74.4 mJ/cm2), leading to a substantial release of trapped Ar atoms for the ion generation. Indeed, the different mechanisms (thermal desorption versus phase explosion) of the ion desorption before and after the threshold laser fluence could also be revealed from the extent of heat transfer from the AuNPs to the chemical thermometer, BP ions, during the laser irradiation, as reflected from the survival yield results (Figure 2c). In the lower laser fluence range below 74.4 mJ/cm2, it was observed that the survival yield of BP ions decreased steadily with increasing laser fluence. The observation in the lower laser 40 fluence was in well agreement with our previous study that the laser-induced heating of the AuNPs could facilitate the thermal desorption of BP ions from the AuNPs surface and inducing the fragmentation of the desorbed BP ions. Nevertheless, when a higher laser fluence was applied, an obvious increase in the survival yield indicating a cooling effect was observed near the threshold laser fluence (~ 74.4 – 82.8 mJ/cm2), which coincided with the laser fluence inducing the abrupt increase in the total ion intensity (Figures 2a and 2b). We believed that phase explosion of the AuNPs induces adiabatic expansion which lowered the internal energy of the AuNP system, and thus leading to the decrease in the extent of heat transfer from the AuNP to BP ions during the ion desorption process. We anticipated that the phase explosion reflected in our mass spectrometric study induced by explosive evaporation, which was also illustrated in our molecular dynamics simulation study below. Molecular Dynamics of the Phase Transition of AuNP upon Laser Irradiation. The ultrafast dynamics of the phase transition of a AuNP (2.5 nm, 487 atoms) irradiated with a pulsed UV laser (at 355 nm for 6 ns duration) in the fluence range of 21.3 mJ/cm2 to 125.9 mJ/cm2 was studied using molecular dynamics (MD) simulation. The physical quantities of the AuNP including potential energies, laser-induced heating temperature and spatial atomic number density ( ρ𝑁 ) were also calculated for rationalization of our experimental results. 12


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Here, the different stages of the phase transition of the AuNP irradiated with a pulsed UV laser at the laser fluence of 125.9 mJ/cm2 were examined in details. The potential energy of the AuNP plotted against the laser irradiation time with corresponding laser-induced heating temperature, is depicted in Figure 3a. In which, the melting point and boiling point of the 2.5 nm spherical AuNP were identified from the abrupt changes in the slopes of the potential energy plot, and were determined to be 835 K and 2,900 K, respectively. As the heating process is in nanosecond time scale, there is not enough time for the laserirradiated AuNP to achieve equilibrium. This resulted in the formation of co-existed solid-liquid phase in a temperature range of 835 K to 900 K and co-existed liquid-gas phase in a temperature range of 2,900 K to 4,400 K. To determine the threshold laser-induced heating temperature for triggering the phase explosion of the AuNP, the number of individual atoms ejected in the ablation plume was extracted and plotted against the laser-induced heating temperature, as shown in Figure 3b. In the figure, four regions divided at 835 K, 2,900 K and 5,800 K were indicated in the plot. Below 2,900 K, the number of individual Au atom ejected is so low that nearly no atoms were observed in the ablation plume. This reflected that the ejection of Au atoms is insignificant when the AuNP was in solid or liquid states. Beyond 2,900 K, vaporization occurred and surface atoms started being ejected from the overheated substrate surface, resulting in a small upsurge in individual atom ablation yield. When the laser-induced heating temperature exceeded 5,800 K, a sudden increase in atom ablation yield was observed. We anticipated that the upsurge of Au atom number ejected could be due to the phase explosion of the AuNP, which could enhance the BP ion desorption efficiency and increase the survival yield via adiabatic expansion. This observation was well agreed with Zhigilei’s computational study on the nickel metal film irradiated by a 1 50 ps laser pulse. It had been reported that such dramatic change in the atom ablation yield 50, 64

is a signal of phase explosion,

and can be explained by nucleation theory.

60, 77-78

The different stages of the phase transition of the laser-irradiated AuNP could also be reflected from the atomic snapshots and the spatial distribution of atomic number density ( ρ𝑁 ) of the AuNP at different laser-induced heating temperature, as illustrated in Figure 4. At 298 K, the AuNP crystal structure is well preserved since the laser-induced heating temperature is far below its melting point at 835 K (Figure 4a). In addition, the Gaussian shape of the atomic number density (shown in the inset of Figure 4a) was due to the spherical shape of the AuNP. When the laser-induced heating temperature was increased to 1,700 K which is beyond the melting point, the Au atoms became mobile and the spherical shape of the AuNP collapsed obviously, as shown in Figure 4b. This resulted in destruction of the lattice structure of the AuNP while there were no Au atoms being 13



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observed beyond AuNP diameter (2.5 nm) as no vaporization occurred (inset of Figure 4b). However, when the temperature was increased to 4,700 K which was much higher than the boiling point (2,900 K), a rapid vaporization occurred (Figure 4c) and metal clusters were ejected from the surface of molten metal and resulted in the detection of large amount of Au clusters beyond 2.5 nm (inset of Figure 4c). At 6,800 K, the laserinduced heating temperature exceeded the phase explosion threshold (~ 5,800 K). As a result, phase explosion occurred and large Au clusters were fragmented in the plume (inset of Figure 4d), and resulted in random and broad spatial distribution of the atomic number density (Figure 4d). The similar trends of potential energies were also observed for the AuNP irradiated at other laser fluence ranging from 21.3 mJ/cm2 to 125.9 mJ/cm2, as shown in Figure S2 of the Supporting Information. However, the maximum laser-induced heating temperature of the AuNP were found to increase with the laser fluence from 21.3 mJ/cm2 (at 520 K) to 125.9 mJ/cm2 (at 16,535 K), and was summarized in Figure S3 of the Supporting Information. Therefore, the magnitude of laser fluence applied could determine the maximum laser-induced heating temperature of the AuNP achieved and, thus affecting the stages of the phase transition involved during laser irradiation. For instance, as shown in Figure S3, when the laser fluence was increased to 29.9 mJ/cm2, the solid AuNP could be heated up to 2,342K, which exceeded its melting point (835K), causing it to melt and transform to liquid state. As the laser fluence was further increased to 45.3 mJ/cm2, the maximum laser-induced heating temperature (2,919 K) was higher than the boiling point (2,900 K) of the AuNP, triggering vaporization and transformation of the AuNP from liquid to gas phase. The maximum laser-induced heating temperature could reach 7,288 K when a laser fluence of 82.8 mJ/cm2 was applied. Phase explosion could occur as this temperature was much higher than the phase explosion threshold (5,800 K). Enhancement of Ion Desorption by Phase Explosion Process. To investigate the effect of the different stages of phase transition of AuNP on the ion desorption efficiency upon laser irradiation, theoretically calculated maximum laser-induced heating temperatures of AuNPs was adopted to correlate with the total intensity of BP ions determined from the mass spectrometric measurement at different laser fluences, as shown in Figure 5. In which, melting point (835 K), boiling point (2,900 K), and phase explosion threshold (5,800 K) of the AuNP were also indicated for the correlation study. As shown in Figure 5, when the applied laser fluence was below 45.4 mJ/cm2, the maximum laser-induced heating temperature was lower than the boiling point (2,900 K) of the AuNP, thus the AuNP was either in solid or liquid state depending on the laser fluence applied, as shown in regions (i) and (ii) of Figure 5. We anticipated that when AuNPs was in solid or liquid state, the desorption of BP ions was mainly driven by the 14


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thermal desorption process and the total intensity of BP ions desorbed from the AuNP remained low, as shown in regions (i) and (ii) of Figure 5. The thermal-driven desorption process was also reflected from the decreasing survival yield of the desorbed BP ions when the laser fluence was increased from 21.3 to 45.4 mJ/cm2 (Figure 2c). When the laser fluence was high enough (at or beyond 45.3 mJ/cm2) to drive the vaporization of AuNPs with the corresponding maximum laser-induced heating temperature (shown in region (iii) of Figure 5) beyond the boiling point (2900 K), a steady increase of the total intensity of BP ions was observed. However, when the laser fluence was further increased to the threshold value at 74.4 mJ/cm2 or beyond (region (iv) of Figure 5), the maximum laser-induced heating temperature achieved were higher than 5,800 K where the onset of phase explosion occurred, and a significant upsurge of the total intensity of BP ions was detected. Therefore, the results revealed that when the applied laser fluence was up to a threshold value or beyond for triggering the phase explosion of the AuNP, the rapid ejection of Au atoms in the ablation plume during the phase explosion could significantly facilitate the desorption of BP ions. Here, we suggested that the weaker binding interaction of BP ions with Au atoms than that with Au clusters and AuNPs might lower the desorption barrier for enhancing the BP ion intensity during the rapid ejection of Au atoms in the phase explosion process. Moreover, it was observed that the total intensity of BP ions increased with laser fluence much more rapidly after the phase explosion threshold and, thus suggesting that phase explosion has the strongest effect in driving desorption of BP ions when compared with vaporization or thermal desorption process.

CONCLUSIONS This study combined experimental SALDI-MS measurement (using a chemical thermometer, BP ions) and molecular dynamics (MD) simulation to rationalize the effects of different stages of substrate phase transition (including melting, vaporization and phase explosion) on SALDI ion desorption, particularly on ion desorption efficiency and the extent of heat transfer. The ion desorption efficiency and heat transfer during the SALDI process upon different laser fluence were measured experimentally in terms of BP ions intensity and survival yield respectively. In addition, the different stages of phase transition were determined computationally from potential energy and ablation plume studies. The maximum laser-induced heating temperature at respecting laser fluence was also calculated and correlated with measured ion desorption intensity and survival yield. The possible physical picture behind the SALDI process, including thermal-driven and phase-transition desorption, was then being discussed based on the correlation results. 15



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Based on the results, it is conceived that SALDI ion desorption could be considered as the control of phase transition stages of substrate upon laser irradiation. Firstly, no BP ions were detected until the laser-induced heating temperature just rose above the melting point and the AuNP started to enter the melting stage. Then, when the AuNP entered the melting stage, ion desorption efficiency slowly increased with laser fluence (i.e. mainly thermal-driven) but the efficiency was generally low. When the AuNP was heated above the boiling point, vaporization of Au atoms occurred and the ion desorption efficiency increased with laser fluence much more rapidly (2–3 fold higher than the melting stage). It is believed that the phase-transition driven desorption started to supersede the role of thermal-driven desorption as the major ion desorption mechanism. Next, an even more obvious jump in ion desorption efficiency was observed (2–3 fold higher than the vaporization stage) when the applied laser fluence exceeded the threshold value for triggering the phase explosion process. Therefore, the significance of the different stages of substrate phase transition for enhancing the ion desorption efficiency could be ranked as: phase explosion > vaporization > melting. Moreover, it was found that the different stages of the phase-transition could also exhibit significant effect on the extent of heat transfer during SALDI process. When the substrate is at solid or liquid state, the survival yield of BP ions decreased continuously with increasing laser fluence, reflecting that the thermal-driven desorption plays an important role in the ion desorption process when the SALDI substrate was in solid or liquid state. In contrast, the phase-transition driven desorption could play a noticeable role to the heat transfer during vaporization, the survival yield of BP ions increased steadily with laser fluence at this stage, revealing that the ablation of SALDI substrate could reduce the extent of heat transfer to BP ions. The effect of phase-transition to the extent of heat transfer was even more apparent when the irradiated laser fluence was high enough to trigger the phase explosion process. In which, the survival yield of BP ions exerted a sudden jump corresponding to an obvious decrease in the heat transfer from AuNPs to BP ions. Such obvious cooling effect to the analyte ions could be rationalized by the rapid adiabatic expansion of the ablation plume. Unlike previous studies which only correlated one/several substrate’s physiochemical properties to the ion desorption efficiency and/or heat transfer, the present study followed the different stages of the substrate under a wide range of laser fluence (21.3 mJ/cm2 − 125.9 mJ/cm2), and rationalized the effect(s) and importance(s) of these properties.

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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (K.M. Ng) Note The authors declare no competing financial interest.

ACKNOWLEGEMENTS We thank Mr. Frankie Y.-F. Chan of the Electron Microscope Unit of The University of Hong Kong and Mr. Y.-H. Cheng for assistance in the size measurements of the Au nanoparticles using transmission electron microscopy. KMNg acknowledges the funding support of the General Research Fund (Grant No.: HKU_17304014) of the Hong Kong Research Grants Council, Seed Funding Programme for Basic Research and Small Project Funding of The University of Hong Kong. We also acknowledge Mr. S.-L. Chau for his preliminary work, and Mr. Cheng for proof read the manuscript.

ASSOCIATE CONTENT Supporting Information Additional details regarding the particle analysis, normalization of ion intensity, effect of laser fluence on the potential energies and laser-induced heating temperature of the AuNP are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Positive SALDI mass spectrum of benzylpyridinium ions ([BP]+ & [BP – Pyridine]+ at m/z 170 & 91 respectively) desorbed from AuNPs substrate at the laser fluence of 52.2 mJ/cm2. Cluster ion series of Au+, Au2+ & Au3+ were also detected at m/z 197, 394 & 591 respectively. (Inset: Detection of K+ & Ar+ at m/z 39 & 40 respectively.)

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Figure 2. Effect of laser fluence (21.3 to 125.9 mJ/cm2) on the (a) normalized total intensity of BP ions; (b) normalized intensity of Ar+ ions; and (c) survival yield of BP ions desorbed from AuNPs. (Each data point is the mean ± standard deviation from 6 – 8 replicate measurements.)


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Figure 3. Effect of laser-induced heating temperature on the (a) potential energies and (b) number of individual gold atoms ablated from a single AuNP (⌀: 2.5 nm) irradiated with a 6 ns UV laser pulse at 355nm and 125.9 mJ/cm2. Melting temperature (835 K), vaporization temperature (2900 K) and phase explosion temperature (5800 K) of the AuNP are labelled by arrows. Only the effect of the first 3.77 ns of 6 ns laser irradiation was shown. The evolution of potential energies as a function of laser-induced heating temperature in the whole laser irradiation duration is depicted as Figure S2 (Supporting Information).

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Figure 4. Effect of laser-induced heating temperature on the average number density of gold atoms and atomic configuration of an AuNP (⌀: 2.5 nm) predicted in MD simulations at a heating temperature of (a) 298 K, (b) 1700 K, (c) 4700 K, and (d) 6800 K. The AuNP was irradiated by a 6 ns laser pulse at 125.9 mJ/cm2 from the right side. The black dashed line represents the diameter of the AuNPs (2.5 nm) used in MD simulations. In general, the AuNP would melt, vaporize and eventually undergo phase explosion when the laser-induced temperature increases.

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Figure 5. Effect of the computed maximum laser-induced heating temperature of the AuNP on the normalized total intensity of BP ions desorbed from AuNPs. The melting temperature, vaporization temperature and phase explosion threshold matched the kinked points of the increasing trends of the normalized total intensity of BP ions. (i) Solid, (ii) liquid, (iii) gas and (iv) phase explosion regions are also labelled.

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Nanosecond UV Laser Ablation of Gold Nanoparticles: Enhancement

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