Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization © The Royal Society of Chemistry 2024 Published version von Witte, Gevin; Himmler, Aaron; Hyppönen, Viivi; Jäntti, Jiri; Albannay, Mohammed M.; Moilanen, Jani O.; Ernst, Matthias; Lehto, Vesa-Pekka; Riikonen, Joakim; Kozerke, Sebastian; Kettunen, Mikko I.; Tamarov, Konstantin von Witte, G., Himmler, A., Hyppönen, V., Jäntti, J., Albannay, M. M., Moilanen, J. O., Ernst, M., Lehto, V.-P., Riikonen, J., Kozerke, S., Kettunen, M. I., & Tamarov, K. (2024). Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization. Nanoscale, Early online. https://doi.org/10.1039/d4nr02603a 2024 Nanoscale PAPER Cite this: DOI: 10.1039/d4nr02603a Received 24th June 2024, Accepted 6th September 2024 DOI: 10.1039/d4nr02603a rsc.li/nanoscale Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization† Gevin von Witte, a,b Aaron Himmler, b Viivi Hyppönen, c Jiri Jäntti, d Mohammed M. Albannay, a Jani O. Moilanen, e Matthias Ernst, b Vesa-Pekka Lehto, d Joakim Riikonen, d Sebastian Kozerke, a Mikko I. Kettunen‡ c and Konstantin Tamarov *‡ d Si nanoparticles (NPs) have been actively developed as a hyperpolarized magnetic resonance imaging (MRI) contrast agent with an imaging window close to one hour. However, the progress in the development of NPs has been hampered by the incomplete understanding of their structural properties that correspond to efficient hyperpolarization buildup and long polarization decays. In this work we study dynamic nuclear polarization (DNP) of single crystal porous Si (PSi) NPs with defined doping densities ranging from nominally undoped to highly doped with boron or phosphorus. To develop such PSi NPs we perform low-load metal-assisted catalytic etching for electronic grade Si powder followed by thermal oxidation to form the dangling bonds in the Si/SiO 2 interface, the P b centers. P b centers are the endogenous source of the unpaired electron spins necessary for DNP. The controlled fabrication and oxidation procedures allow us to thoroughly investigate the impact of the magnetic field, temperature and doping on the DNP process. We argue that the buildup and decay rate constants are independent of size of Si crystals between approximately 10 and 60 nm. Instead, the rates are limited by the polarization transfer across the nuclear spin diffusion barrier determined by the large hyperfine shift of the central 29 Si nuclei of the P b centers. The size-independent rates are then weakly affected by the doping degree for low and moderately doped Si although slight doping is required to achieve the highest polarization. Thus, we find the room temperature relaxation of low boron doped PSi NPs reaching 75 ± 3 minutes and nuclear polarization levels exceeding ∼6% when polarized at 6.7 T and 1.4 K. Our study thus establishes solid grounds for further development of Si NPs as hyperpolarized contrast agents. 1 Introduction Magnetic resonance imaging (MRI) is a non-invasive versatile technique that can provide anatomical images with either submillimeter spatial 1 or milliseconds temporal 2 resolution. Applying recent advances in artificial intelligence based image reconstruction and enhancement methods, 3 low field MRI has recently reached real world adoption even in the mobile setting. 4 MRI, however, is inherently insensitive at room temperature due to low thermal polarization of nuclei, which complicates the observation of nuclei other than 1 H. Detecting lowabundant nuclei, such as 13 C, 15 Nor 29 Si, brings additional versatility to MRI allowing to e.g., image tumor metabolism, 5 locally detect pH, 6,7 detect Si particles in vivo during prolonged time window. 8–10 Porous Si nanoparticles (NPs) hold particular promise due to their biocompatibility and numerous treatment modalities. 11 To detect Si NPs in an MRI scanner, their 29 Si nuclei require hyperpolarization i.e., a polarization significantly beyond thermal equilibrium at body temperature. A mature method to hyperpolarize various nuclei in the solid state is dynamic nuclear polarization (DNP). 12 DNP requires the †Electronic supplementary information (ESI) available: FTIR, XRPD, EPR characterization data for all the samples as well as the additional DNP data including DNP sweep spectra, polarization buildup and decay data, rate constants for various experimental conditions. See DOI: https://doi.org/10.1039/d4nr02603a ‡Joint senior authorship. a Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland b Institute of Molecular Physical Science, ETH Zurich, Zurich, Switzerland c Kuopio Biomedical Imaging Unit, A.I. Virtanen Institute, University of Eastern Finland, Kuopio, Finland d Department of Technical Physics, University of Eastern Finland, Kuopio, Finland. E-mail: kons[email protected] e Department of Chemistry, Nanoscience Center, University of Jyväskylä, Jyväskylä, Finland This journal is © The Royal Society of Chemistry 2024 Nanoscale Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal presence of polarized unpaired electronic spins, whose polarization is subsequently transferred to hyperfine (HF) coupled nuclei by (near-) resonant microwave (MW) irradiation. In Si, the unbound electrons required for DNP can originate from substitutional donor dopant atoms, such as group V (P, As, Sb, Bi) or group VI (S) atoms, which carry one or more donor electrons. As each dopant carries extra electron(s), the majority carriers are negatively charged electrons and Si is named n-type. Spins of electrons bound to 31 P donors have been widely used to polarize 29 Si nuclear spins and to study polarization dynamics in bulk Si samples with different 29 Si and 31 P content. 13–16 With the variation of 31 P and 29 Si content, well resolved solid effect (SE), 13,15,16 differential SE 13 and Overhauser effect (OE) 14–16 DNP mechanisms of 29 Si hyperpolarization have been identified. More sophisticated protocols, such as resonant polarization transfer from polarized 31 Pto 29 Si nuclei, 17 have been demonstrated. If Si is doped with group III atoms, in particular boron, each dopant atom binds an electron leaving a hole in the valence band. The majority carriers are the positively charged holes and the Si is called p-type. Hole states in the valence band from the p-orbitals as opposed to the s-orbitals of electrons in the conduction band. The need to satisfy the 3-fold degeneracy of the p-orbital results in the splitting of the valence band into heavy and light hole bands. 18 The degeneracy of these bands combined with the dopant atom-induced local random stresses broadens the electron paramagnetic resonance (EPR) spectrum making it hard to observe in B-doped Si unless uniaxial strain is applied. 19–21 Strained single crystal Si : B has been used to study the integrated solid effect. 20 Another source of electron spins are defect sites found in amorphous Si, 22 oxidized Si surfaces 23–25 and elemental Si particles. 8,9,26 Such defect sites are characterized by a broken Si bond with an unpaired electron mostly localized on the central Si atom. 25 When the defect is located at the Si/SiO 2 interface, it is called the P b center. 23–25 P b centers and P b -like centers in amorphous Si have been used to hyperpolarize various Si particles and apply them as background-free contrast agents for MRI. 8,27 The long hyperpolarization decay times (τ dec ) of Si particles around ∼40 min at room temperature offered extended imaging time windows compared to about 30 s in 13 C molecules 6 or 145 s (15 min) in nanodiamonds (microdiamonds). 28 In diamonds, the substitutional nitrogen defects in the particle’s bulk (often called C or P1 center) are responsible for DNP while surface dangling bonds commonly cause strong relaxation. The surface dangling bonds thus are detrimental for nanodiamonds with high surface-to-bulk ratio leading to lower polarization levels and faster relaxation compared to microdiamonds. 29 This is different from the case in Si with the P b centers located on the interface to the naturally forming surface oxide which allows the hyperpolarization of 50 nm particles with identical enhancements compared to μm-sized particles. 26 Despite the demonstrated high nuclear polarization and long nuclear τ dec relaxation times at room temperature 26,30 in bulk Si particles, the proposed slow spin diffusion fails to explain the similar τ dec in micro- and nanoparticles. The diversity of fabrication methods further complicates the identification of the structural properties, their comparison between different particles and influence on τ dec .Inthisstudy,weapplya top-down fabrication approach 31,32 to produce porous silicon nanoparticles (PSi NPs, sometimes denoted as nanobeads) with a high surface area from doping controlled, single crystal Si wafers. The role of the high surface area is twofold. First, it enables the controlled formation of a relatively large number of endogenous P b centers to drive the DNP process. To the best of our knowledge, previous attempts to hyperpolarize PSi NPs required the use of external radicals for DNP to be efficient 10 complicating possible MRI applications of those NPs. Second, the large surface area combines good biocompatibility with a well understood diverse chemistry for (targeted) nanocarrier capabilities 11 making the developed PSi NPs suitable both for imaging and drug delivery. 11 Herein, we prove that endogenous P b centersinPSiNPsare capable of providing DNP enhancements similar to state-of-the- art bulk particles. 26 Furthermore, we demonstrate that PSi nanoparticlesfromslightlydopedSiwaferscanachieveroomtempera- ture hyperpolarization decay times exceeding one hour and 29 Si polarization levels around 6%. 2 Methods 2.1 Silicon Previous studies on the DNP of Si NPs relied on either commercially available 9,26,30,37 or on in-house bottom-up fabrication approaches. 10,30,38,39 In contrast, we selected single crystal Si wafers as the starting material to precisely control crystallinity and doping level (Table 1). Specifically, we used elec- Table 1 Summary of Si grade abbreviations used to fabricate PSi NPs Abbreviation Resistivity (Ωcm) Dopant Doping density a (cm −3 ) Average dopant distance b (nm) P++ c 0.0186 Boron 4 × 10 18 3.49 P+ c 0.105 Boron 3 × 10 17 8.27 P c 25 Boron 5 × 10 14 69.8 UW c >5000 Boron <10 12 >554 N c 19.7 Phosphorus 2.3 × 10 14 90.4 N+ c 1.15 Phosphorus 3 × 10 15 38.4 N++ c 0.0144 Phosphorus 3 × 10 18 3.84 MC10 d Metallurgical grade powder, polycrystalline, 99.997% purity. Impurities: Al, Fe, Ca, Ti a Dopant densities were calculated using Caughey–Thomas expression 33 for electron and hole mobilities. Effective Bohr radii are 1.3 (3.8) and 2.1 nm for heavy (light) holes and electrons in B doped and P doped Si, respectively. The effective Bohr radius of the P electron assumes the pancake-like wavefunction ansatz proposed by Kohn and Luttinger. 34 b Average distance between the dopant atoms was calculated from their density using the random probability distribution in three dimensions. 36 c Powder from single crystal (100) wafers, Okmetic. d Elkem Silicon Products. Paper Nanoscale Nanoscale This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online tronics grade single crystal (100) silicon wafers of different doping (Okmetic Oy, Finland). The samples were denoted according to the doping type and doping density. Doping type was indicated by P (positive) and N (negative) letters for boron and phosphorus doping, respectively. The doping density ranged from 4 × 10 18 cm −3 for P++ and N++ porous Si (PSi) NPs down to less than 10 12 cm −3 for the nominally undoped wafer (UW). The doping density was below the insulator-to- metal transition for all Si wafers considered. In addition to wafers, we prepared PSi NPs from a relatively cheaper commercially available polycrystalline (1–10) μm Silgrain Supreme MC10 SB powder (Elkem Silicon Products, Norway) with known concentration of impurities (MC10 sample): the purity of the powder was 99.997% determined by the supplier, where the main impurities were Fe (14 ppm), Al (6 ppm), Ca (3 ppm), Ti (1 ppm), B (<1 ppm), and P (<1 ppm). Dopant type of the Si wafers was verified by hot point probe method. 40 Specific resistivity was calculated using a MATLAB (The MathWorks, Inc., USA) script using wafer thickness and resistivity measured with a four-point probe (Jandel Engineering Ltd, UK) connected to a Cropico DO5000 microhmmeter (Seaward Electronics Ltd, UK). 40 The dopant concentrations were estimated by comparing the measured specific resistivities with the ones calculated using Caughey–Thomas expression 33 from electron and hole mobilities at 300 K assuming full ionization of dopant atoms. The average distances between dopant atoms were calculated from the doping densities using the probability density function to find the atom at a distance r. 36 Assuming the uniform random distribution of the dopant atoms, the average distance is 〈r〉≈ 0.554·N c− 1 3 , where N c is the density of atoms. The equivalent Bohr radii for acceptors were estimated using the expression derived from the hydrogen atom-like model of donor (acceptor): a A =ε r m 0 /m eff ·a 0 , where a 0 is the Bohr radius of hydrogen atom, ε r is the relative dielectric permittivity of Si, m 0 is the electron mass, and m eff is the effective mass of a hole. For the donors, amore precise value of the electron’seffective Bohr radius is given by the geometric mean a D =a 1 3 ka 2 3 ?≈2.087 nm, where a ∥ ≈1.44 nm and a ⊥ ≈2.51 nm are the two radii of the pancake-like wavefunction ansatz for the electron ground state proposed by Kohn and Luttinger. 34,35 The information about Si types and abbreviations used in the text are summarized in Table 1. 2.2 Porous Si powders (10–25) μm powders were prepared by ball-milling Si wafers using Fritsch Pulverisette 7 Premium Line (Fritsch GmbH, Germany). Obtained powders were washed in 3 wt% aqueous H 2 O 2 by sonicating them for 1 h in an ultrasound bath. 32 Such washing removes possible surface contaminations and ensures reproducibility. The powders were then dried and used to produce porous Si by low-load metal-assisted catalytic etching (LL-MACE) as reported before. 31,32 The protocol was scaled up to perform etching of 2 g Si powder batches. Briefly, 2 g of Si powder was first dispersed in 30 ml of acetic acid (Ph. Eur., VWR Chemicals) inside of a 50 ml PTFE dish by 5 min sonication. Then, 30 ml of hydrogen fluoride (HF, 30–40%, Merck) was added, and the dish was placed in a water bath on a heat plate under stirring. Next, Au NPs were nucleated on Si powder surfaces using a syringe pump injection of 8.334 ml (= 50 μmol) of 0.006 M Au 3+ ion solution, which was prepared by dissolving HAuCl 4 ·3H 2 O (99.99%, Alfa Aesar, Thermo Fisher GmbH) in water. Injection rate was 333.3 μl min −1 ; after it was completed, Si powder suspension was stirred for 5 min more to complete the nucleation of Au 3+ NPs. The temperature of the water bath was kept at about 39 °C to retain the temperature of the suspension in the range of (51–53) °C during etching. The etching was performed by injecting H 2 O 2 /H 2 O solution using the syringe pump at a rate of 133.3 μl min −1 (injection time equals to 90 min). The H 2 O 2 volume (35 wt%, Acros Organics, Thermo Fisher GmbH) in the solution was selected to match the H 2 O 2 /Si molar ratio of 1.03. The open end of the plastic tube going from the syringe was immersed into the suspension with Si particles. After the etching finished, porous Si particles were washed in Büchner-style funnel on a 55 mm diameter Grade 2 WhatmanRRR filter. After the etching solution was removed, porous Si particles were washed three times on the filter using deionized water. Next, about 10 ml of n-pentane (≥99%, VWR Chemicals) was poured on the filter with porous powder and it was allowed to dry for a few min under the fume hood. N-Pentane reduced the surface tension of water inside the pores and prevented the collapse of porous structure during the final drying which was completed overnight in an oven at 65 °C. Obtained microscale PSi powders were then stored in glass vials. 2.3 Surface oxidation and preparation of nanoparticles After LL-MACE surfaces of PSi powders were hydrogen terminated (Fig. S3 and S4, ESI†). Localized electronic defects (P b centers) formed at the Si/SiO 2 interface during thermal oxidation of PSi particles. This approach gives the highest number of P b defects among other methods. 41 Thermal oxidation was done for 2 hours in NaberTherm R50/500/12 tube furnace (Nabertherm GmbH) at 345 °C in air. 41 Thermally oxidized PSi powders were then milled into NPs using a dedicated system. 42 About 400 mg of a PSi powder was placed into a 4 ml glass vial which was subsequently filled with 1 mm ZrO 2 milling balls. The milling was then performed in 5 min cycles at 900 rpm to avoid overheating and leaks. After each cycle, pressure was released from the vials. Typically, 10 cycles were enough to obtain PSi NPs with most of the particles below 200 nm in hydrodynamic diameter (Fig. 1b and S1, ESI†). In addition to thermal oxidation, two-step liquid-phase oxidation (oxidation in H 2 O 2 /NH 4 OH solution followed by oxidation in H 2 O 2 /HCl solution), 41 or one-step (only H 2 O 2 /HCl solution) was performed for thermally oxidized PSi NPs (i.e., after milling of thermally oxidized PSi powders, details in ESI†). Liquid-phase oxidation removed the remaining hydrogen in −O y SiH x groups (Fig. S3 and S4, ESI†) as well as induced additional backbond oxidation. Liquid-phase oxi- Nanoscale Paper This journal is © The Royal Society of Chemistry 2024 Nanoscale Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online dation was tested because it would be needed in future surface modification with PEG molecules to prolong the systemic circulation time and enabling the use of the PSi NPs e.g., as MR imaging agents. 43 2.4 Au removal The absence of Au NPs influence on the DNP was confirmed with the N sample. Au NPs were dissolved using the KI/I 2 gold etchant for the porous Si powder after LL-MACE. Gold etchant solution was prepared by dissolving 6.08 g of KI and 1.51 g of I 2 in 30 ml of 5 M HCl. Use of HCl as solvent demonstrated better Au dissolution compared to water. To dissolve Au NPs, about 250 mg of N powder after LL-MACE was dispersed in 3 ml of ethanol to wet the hydrophobic surfaces. Then, 15 ml of gold etchant was slowly added to the Si powder suspension. Particles were then stirred for 2 h at 75 °C. Au amount before and after the dissolution was measured using a home build portable XRF setup 44 and calculated using the calibration standards prepared with Au deposition step of LL-MACE. Finally, particles were washed 3 times with water in a Büchner-style funnel, wetted with n-pentane and dried in an oven as above. Then the powder was milled to NPs and denoted as N–Au. 2.5 Characterization Morphology of PSi NPs was examined by transmission electron microscopy (JEOL JEM-2100F, JEOL Ltd). A 2.5 μl drop of suspension diluted in ethanol to a concentration of 20 μgml −1 was dried on 400 mesh Cu holey carbon grid (Agar Scientific Ltd) and the grid was examined in the instrument. Hydrodynamic sizes of PSi NP were measured using dynamic light scattering (Zetasizer Nano ZS, Malvern Panalytical Ltd) after redispersion in water at 0.1 mg ml −1 concentration. Specific surface area, specific pore volume and pore size distributions of PSi powders after LL-MACE were characterized by N 2 sorption (Tristar II, Micromeritics Instrument Corp.). Specific surface areas were calculated from the linear part of adsorption isotherm using Brunauer–Emmett–Teller theory. Specific pore volumes were obtained from the total adsorbed amount at relative pressure of 0.97. Pore size distributions were calculated from desorption isotherm using Barrett– Joyner–Halenda model. Pore sizes and sizes of catalytic Au NPs were further measured with X-ray powder diffraction (XRD, D8 Discover, Bruker Corp.). Powders were placed on a zero-background holder and scanned in (25–60)° two-theta angle range with step size of 0.0057° and time per step of 0.205 s. Crystalline sizes of two Si phases and one Au phase were then calculated with Rietvield refinement method using TOPAS® 4.6 software (section S2.3, ESI†). The sizes calculated from the Si peak broadenings corresponded to the two pore sizes according to the Babinet’s principle in single crystals. 45 Surface chemical species and P b centers formed by oxidation were studied with Fourier-transform infrared spectroscopy (FTIR, Thermo Nicolet iS50, ThermoFisher Scientific Corp.) and electron paramagnetic resonance spectroscopy (EPR, Magnettech MiniScope MS5000, Bruker Corp.). In FTIR, KBr tablets with dried PSi NPs were measured in transmission mode (Sec. S2.2, ESI†). For EPR measurements, the first 7 mm of an EPR tube were filled with dried PSi NP powder. The tube was placed in the spectrometer at the same height for each Fig. 1 Characterization of PSi NPs. (a) Typical transmission electron microscopy image of PSi NPs dried out of suspension; the inset shows the high magnification view. (b) Hydrodynamic size distribution of the N PSi NPs in water suspension. (c) Pore size distribution of N Si powder after LL-MACE. (d) Specific surface areas and pore volumes obtained from N 2 sorption measurements of Si powders after LL-MACE. (e) Crystalline sizes of Au NPs and pore sizes in PSi NPs calculated from X-ray powder diffraction spectra. (f) Electron paramagnetic resonance spectrum of N PSi NPs. The experimental data (black circles) was fitted (red lines) using trigonal Pð111Þ band isotropic Piso bcenters (details see text and section S2.4, ESI†). (g) P b defect density of PSi NPs formed by thermal oxidation (no label), thermal and two-step liquid-phase oxidation (2LO label), thermal and one-step liquidphase oxidation (1LO label), and oxidation induced by Au dissolving solution (–Au label). Paper Nanoscale Nanoscale This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online measurement with the following parameters: (1) B 0 = 336 mT, Bscan 0= 15.5 mT, Bmodulation 0= 0.2 mT, t scan = 60 s, MW attenuation 25 dB and gain 10 for the full spectra; (2) B 0 = 336 mT, Bscan 0=35mT,Bmodulation 0= 0.7 mT, t scan = 60 s averaged 3 times, MW attenuation 15 dB and gain 500 to resolve hyperfine peaks. To calculate the concentration of P b centers and the g-factor, a standard 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) radical (99%, Sigma-Aldrich) sample with known number of paramagnetic centers and g-factors was used. EPR spectra were fitted using EasySpin 5.2.35 by simulating solidstate continuous-wave powder spectra using a combination of anisotropic Pð111Þ band isotropic Piso bcenters. g-Factor strain, hyperfine coupling and Voigtian line broadening were included (section S2.4, ESI†). 46 2.6 Dynamic nuclear polarization Hyperpolarization of PSi NPs was studied using three different polarizer designs: SpinAligner (Polarize ApS) operating at 3.35 T or 6.7 T and a base temperature of 1.4 K as well as with two home-built setups with 3.34 and 7 T (ref. 47–49) with both operating at a base temperature of 3.4 K. About 100 mg of dried PSi NP powder was packed into a polymer sample container for measurements with the SpinAligner compared to (50–60) mg for the home-built set-ups. Microwave radiation was delivered through a waveguide elbow to directly irradiate the sample. The microwave irradiation 9,26 was frequency modulated in all polarizers. Magnetic field strength, temperature, microwave power W, frequency modulation bandwidth Δν MW and frequency of modulation ν MW are summarized in Table 2. To monitor the 29 Si signal, a flip angle of ∼2.8° was used in the SpinAligner with varied time intervals between the measurements. Flip angles of ∼1.5° each 20 min at 3.34 T and ∼6.9° every 6 to 10 min at 7 T were used. Obtained data was analyzed using MATLAB scripts, where either the real part of the time-domain free induction decay (FID) was fitted with an exponential ansatz or the real part after fast Fourier transform (FFT) in the frequency-domain was fitted with pseudo-Voigt functions. Polarization enhancements and absolute polarization values were calculated from the thermal equilibrium signal taken in the hyperpolarization conditions after 72 h of polarization with microwave irradiation switched offfor the 6.7 T (1.4 K) measurements (section S3.1, ESI†). For the 3.34 and 7 T (3.4 K) measurements, the thermal equilibrium signal at 300 K of a fully 29 Si isotope labeled sample (Isoflex, Russia) was measured and adjusted for temperature upon calculation of enhancements and absolute polarization in the DNP experiments. Both the polarization buildup data and the relaxation data was corrected for the perturbations by the monitoring RF pulses 50 (except for the 3.34 T due to the small flip angle used and difficulties in measuring such small flip angles with high relative accuracy). 2.7 Correlation analysis Correlation analysis was performed using the Matlab corrcoef() function. The data supplied to the function consisted of sample characterization data (Fig. 1), EPR data obtained from fitting (Tables S1 and S2, ESI†), DNP data obtained from fitting and the rate-equation model (Fig. 3 and S26, ESI†). 3 Results The applied fabrication procedure (Methods section) results in irregular shaped PSi NPs with average particle sizes of (150 ± 65) nm (Fig. 1a and b). Additional milling and centrifugal selection could further reduce particle sizes if required for a specific (biological or medical) application (section S2.1, ESI†). The porous structure with two distinct pore sizes was formed during the Au-catalyzed LL-MACE (Fig. 1c). Etch track pores (>10 nm) were produced by Au NPs boring into Si, while tortuous pores (<10 nm) were formed by hole escape from spacecharge layers to distant Si surfaces. 31,32 This porosity resulted in a high surface area and a high number of surface P b centers after oxidation (Fig. 1d and g). X-ray powder diffraction (Fig. 1e and section S5, ESI†) showed Si peaks with distinct superimposed peak profiles. Typically, the peak broadening of small crystals is dependent on the crystallite size but the porous nature of the PSi NPs complicates the picture. Since the Si particles are single crystals before etching (except for the MC10 sample) and preserve the crystallinity during the etching, the pores also give a contribution to the peak broadening according to the Babinet’s principlecite. 45 Therefore, three distinct contributions to the peak broadening would be expected for the PSi NPs caused by the crystallite size, population of wide etch track pores and the population of narrow tortuous pores. It was not possible to reliably fit the data with three peak profiles but instead fit with two profiles was done. The narrower peak profiles ((30–60) nm bars) were attributed to the etch track pores and the broadening from the small crystal size, while the wider ones ((5–10) nm bars) were due to tortuous pores penetrating the large crystals (Fig. 1e). 31,32 The wide and narrow XRD peaks were on the order of the corresponding pore sizes measured by N 2 sorption and depicted in Fig. 1c. The subsequent thermal oxidation (Methods and section S2.2, ESI†) stabilized the H-terminated surface of freshly etched samples simultaneously making them hydrophilic. The created core–shell structure of PSi NPs thus consisted of the crystalline cores of pore walls (bulk) with a thin oxide shell Table 2 Summary of the DNP conditions B 0 ,T T,K W a ,mW Δν FM b , MHz ν FM c , kHz 3.34 3.4 200 ∼150 1 3.35 1.4 80 100 1 6.7 1.4 30 200 3 7 3.4 200 d 300 10 a Microwave power. b Frequency modulation bandwidth. c Frequency of the modulation. d Silver-plating the waveguide approximately doubled the MW power reaching the sample 49 for the nominal 200 mW output of the source. Nanoscale Paper This journal is © The Royal Society of Chemistry 2024 Nanoscale Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online (surface). The lattice constant mismatch between the Si and SiO 2 led to the formation of paramagnetic centers in the Si/ SiO 2 interface. Electron paramagnetic resonance (EPR) spectra (Fig. 1f, discussion of P b centers below and section S2.4, ESI†) showed the presence of two typical paramagnetic centers found on oxidized (porous) Si surfaces: (i) trigonal Pð111Þ bcenters with axial symmetry similar to defects found in oxidized planar (111) and porous Si surfaces (g ∥ = 2.00185, g ⊥ = 2.0081) 25,51,52 and (ii) isotropic Piso bdefects commonly observed in oxidized porous Si (g= 2.0055). 51,53–57 EasySpin 58 was used to simulate the experimental EPR spectra to obtain the relative weights of the Pð111Þ band Piso bcenters in our samples (section S2.4, ESI†). The simulations gave typical weights of (10–20)% for the Pð111Þ b and (80–90)% for the Piso bdefects. It was expected that Piso bis the dominant defect center due to the random nature of pore formation in LL-MACE and thermal oxidation in air. Hyperfine (HF) interaction with the central 29 Si was also observed (section S2.4, ESI†) and measured to be in the range of A= (325–431) MHz, which coincided well with A ∥ = 210 MHz and A ⊥ = 417 MHz for the planar Pð111Þ bcenter. 52 The number of all types of P b centers per unit area and per mass varied between (1.8–6.8) × 10 12 cm −2 (Fig. 1g) and (4.4–6.3) × 10 15 mg −1 , respectively (section S2.4, ESI†). These values corresponded to the fraction of total P b centers per silicon interface atoms of f ≡[P b ]/N a = (0.23–0.87)% (where N a = 7.83 × 10 14 cm −2 is the density of lattice sites in the (111) plane). The average distance between the P b centers was then calculated from the concentration per unit area using the nearest neighbors distribution 36 derived for the 2D case. The average distances varied between 1.9 nm (N 1LO PSi NPs) and 3.7 nm (N++ PSi NPs). Correspondingly, the maximum dipolar interaction between electron spins of P b centers ranges from 1.0 to 7.4 MHz if a uniform surface distribution of P b centers is assumed. We performed DNP NMR hyperpolarization and relaxation studies at four different conditions (3.34 T and 7 T at 3.4 K, 3.35 T and 6.7 T at 1.4 K, Table 2) with only selected samples evaluated at all the experimental conditions. The measured DNP profiles followed the symmetry of the EPR spectrum with the positive and negative DNP lobes located at a similar distance to the central zero crossing of the DNP enhancement (section S3.3, ESI†). The zero crossing of the DNP enhancement coincided with the center of the EPR line in agreement with previous works with endogenous defects in Si. 26,30 29 Si polarization buildup data at 6.7 T (1.4 K) for the thermally oxidized PSi NPs with various dopants are depicted in Fig. 2. The data was corrected for the perturbations by the monitoring RF pulses. 50 We confirmed that the algorithm correctly recovered the genuine buildup dynamics from high sampling rate data in Fig. 2 using a low sampling rate of 30 min for the P sample (Fig. S17, ESI†). The one-compart- ment model underlying the RF correction assumes a monoexponential buildup and decay dynamics 50 as observed in all our samples and experimental conditions (section S3.3, ESI†). The polarization buildup (at 6.7 T and 1.4 K) depended on the doping degree. The lowest polarization was found for the highly doped P++ and N++ samples but with significant difference between them despite the similar doping level of the Fig. 2 Dynamic nuclear polarization of thermally oxidized PSi NPs with different dopants after correcting for perturbation by the RF pulses 50 (dark squares) and single exponential fit (green lines) at 6.7 T and 1.4 K. The microwave frequency was set to 187.82 GHz with a 150 MHz modulation bandwidth, 3 kHz modulation frequency and 30 mW microwave output power. The enhancement is relative to the thermal polarization of 29 Si nuclear at the polarization buildup conditions. For the characteristics of the various samples, see Table 1. Paper Nanoscale Nanoscale This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online starting Si powder (Table 1). With the decrease of doping density, the gained polarization levels tended to equalize between different doping types (P+ and N+ PSi NPs). Interestingly, the nominally undoped UW PSi NPs did not show the highest absolute 29 Si polarization; the highest polarization levels were obtained for lightly doped P and N samples. Moreover, the relatively impure polycrystalline MC10 PSi NPs showed slightly better DNP polarization and similar buildup times than moderately doped P+ and N+ samples. Such polycrystalline grades could thus be a cheaper alternative to electronics grade sample with sufficiently good DNP properties. The DNP characteristics changed significantly at 3.34 T and 3.4 K (Fig. 3 and S18, ESI†). The polarization buildup times (Fig. 3c) for all the samples almost halved compared to 6.7 T (1.4 K). The observed enhancements (Fig. 3b) were significantly higher at 3.34 T especially for the low B doped PSi NPs compared to the 6.7 T data. The n-type samples demonstrated only moderate enhancement increases with the N sample showing even lower enhancement than at 6.7 T. Despite the higher enhancements at 3.34 T, the estimated absolute 29 Si polarization was still higher at 6.7 T (1.4 K) compared to 3.34 T (3.4 K) (Fig. 2, 3a and S18, ESI†) due to the higher thermal nuclear polarization. In order to clarify the influence of the experimental conditions on DNP, we performed selected measurements at 7 T (3.4 K) to discriminate between field and temperature dependent changes (Fig. 3 and S19, ESI†). The decreased polarization for the N PSi NPs clearly followed the same trend as at 3.34 T while the absolute enhancement values and buildup times for P and UW samples were close to the 6.7 T data. The similarities for P and UW samples were even more striking provided the MW power was 30 mW at 6.7 T compared to 200 mW at 7 T. We then verified at 7 T (3.4 K) that 200 mW and 20 mW provided similar enhancements at 7 T making the comparison between 6.7 T and 7 T possible despite the large difference in MW power (Fig. S22, ESI†). We, therefore, conclude that temperature plays the crucial role in DNP performance of n-type PSi NPs, while it has less influence on the p-type samples. The temperature dependence for p-type samples was further investigated at 3.35 T (1.4 K) (Fig. S23, ESI†). We found a significant decrease of enhancement levels compared to the other conditions with minor differences between P and P++ PSi NPs. In addition to the thermal oxidation used to create P b centers on differently doped PSi NPs, we applied liquid oxidation 41 to the P and N PSi NPs. Liquid oxidation reduced the number of surface hydrogen in –Si y H x -Si–H and –O 3 SiH surface groups (section S2.2, ESI†), which is an important step towards an improved surface coating for biomedical applications. 43 We note that liquid oxidation affected the p- and n-type Si samples differently (section S2.2, ESI†). The same is true for the measurements with different DNP conditions (Fig. 4 and Fig. S18, S20, S21, ESI†): for the P sample, enhance- Fig. 3 Comparison of 29 Si nuclear polarization (a), enhancement over the thermal signal (b) and polarization buildup time (c) for PSi NPs at 6.7 T (1.4 K) (orange bars) as well as 3.34 T (3.4 K) (green bars) and 7 T (3.4 K) (violet bars). Temperature decrease or increase of magnetic field strength increase the thermal nuclear polarization used to calculate the enhancement from the nuclear polarization. The polarization, enhancement and buildup time are corrected for perturbations by the RF pulses. 50 MW frequency modulation was employed in all the experiments. Fig. 4 Relative change of the 29 Si steady-state polarization (enhancement) (a) and polarization build up time (b) due to oxidations for P and N samples. The 2LO oxidation indicates the two-step liquid oxidation (section S1.1, ESI†) performed after the thermal oxidation. For the N–Au, oxidation emerged during the Au removal after LL-MACE (Experimental section). The dashed line indicates no change i.e., the same measured value compared to thermally oxidized samples. The absolute values are in Fig. S21, ESI.† Nanoscale Paper This journal is © The Royal Society of Chemistry 2024 Nanoscale Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online ment dropped significantly at 6.7 T (1.4 K) and 3.34 T (3.4 K). Contrary to the P sample, liquid oxidation of the N sample increased the enhancement about 1.4 times at 3.34 T (3.4 K), while at 6.7 T (1.4 K) the enhancement decreased. The polarization build up times were affected in a more consistent manner (Fig. 4b): for all the samples and liquid oxidations, the buildup times shortened to (0.5–0.7) times the buildup time of the thermally oxidized N or P PSi NPs. Future studies might explore the influence of oxidation, doping, and DNP conditions on the DNP via P b centers further. Finally, we verified that the presence of Au NPs left in PSi NPs after LL-MACE had little impact on DNP performance. For verification, we applied an iodine-based Au etchant to the N PSi NPs directly after LL-MACE (no thermal oxidation). The Au dissolution resulted in decrease of Au content from 0.37% for N PSi NPs to 0.02% for N–Au PSi NPs as measured by XRF. Since the Au etchant is a strong oxidative solution, the dissolution process also oxidized the PSi NP surfaces which are hydrogen terminated and hydrophobic after LL-MACE. For N PSi NPs, the etchant-induced oxidation had similar effects as liquid oxidation (Fig. 4). After collecting the DNP data for the various samples at 3.34 T and 6.7 T, we selected the P, UW and N samples for room temperature relaxation measurements (Fig. S25, ESI†). For this, the samples were hyperpolarized at 3.34 T (3.4 K) for around 20 h and subsequently transferred (dry, tightly packed sample container) to the nearby temperature-controlled (300 K) 7 T setup. At room temperature, the differences between the decay times τ dec of the selected samples diminished compared to liquid helium temperatures (Table S3, ESI†). Nevertheless, a smaller τ dec for the N sample compared to the P and UW samples was observed. The hyperpolarized decay times at room temperatures of the P and UW samples were around 70 min (Table 3). 4 Discussion The following discussion is organised along Fig. 5, which sketches the relevant length scales and the proposed polarization pathway in the PSi NPs. The P b centers at the interface between the surface oxide shell and the crystalline silicon core provide the unbound electrons required for DNP. Thus, understanding DNP in PSi NPs requires a basic understanding of P b centers, which will be provided in section 4.1. To achieve a hyperpolarized nuclear state, the high thermal electron polarization is transferred via hyperfine (HF) coupling to 29 Si nuclei of a P b center located on the interface between the bulk pore walls and oxide shell (step 1 of hyperpolarization buildup sketched in Fig. 5). The HF coupling shifts the resonance frequency of the P b nuclear spins rendering it difficult to observe these spins with NMR (hypershifted spins 59 ). The hypershifted spins have a resonance frequency (energy) discrepancy to the bulk nuclear spins in the pore walls (visible by NMR). The frequency discrepancy suppresses the nuclear spin diffusion between the hypershifted and bulk spins (step 2 in Fig. 5) making the step to be the time limiting as further argued below in section 4.2. Between the bulk 29 Si in the pore wall cores, nuclear spin diffusion (nSD) spreads the nuclear hyperpolarization throughout the crystalline pore wall cores (step 3 in Fig. 5). The discussion of the different steps is then extended to the hyperpolarization decay in section 4.3. Finally, the effects of different samples and experimental conditions are discussed in section 4.4. 4.1 P b centers In section S2.5, ESI,†we concisely review existing literature on the interfacial P b centers in Si/SiO 2 . Based on this review, the measured EPR spectra are fitted with two types of P b centers: (i) a Pð111Þ bcenter with trigonal symmetry and (ii) Piso bwith spatially isotropic g-factor. If a P b center has a 29 Si nucleus at its central site, it possesses a Fermi-contact (isotropic) hyper- Table 3 Relaxation time of the selected PSi NPs at 7 T and room temperature after DNP at 3.34 T and 3.4 K Abbreviation τ dec , min P75±3 UW 67 ± 8 N52±5 Fig. 5 Sketch of the PSi NPs with ∼150 nm particle size and a large number of torturous pores (not to scale). P b centers form at the interface between the surface oxide shell and the crystalline pore wall cores. Average electron–electron (r ee ) and ( 29 Si) nuclear-nuclear (r nn ) distances for 4.7% natural abundance 29 Si are indicated. The hyperpolarization pathway is indicated in red. The polarization is transferred from the electron to the hypershifted 29 Si (h) nucleus of a P b center (step 1) and from there to a nearby bulk (b), NMR visible 29 Si spin (step 2). Within the crystalline pore wall core (step 3), the nuclear hyperpolarization is spread via nuclear spin diffusion. Only 4.7% of P b centers have 29 Si nucleus and, therefore, directly participate in DNP. Paper Nanoscale Nanoscale This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online J. Salonen and V. P. Lehto, Langmuir, 2012, 28, 10573– 10583. 42 T. Nissinen, T. Ikonen, M. Lama, J. Riikonen and V.-P. 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Mater., 2019, 31, 678–688. Nanoscale Paper This journal is © The Royal Society of Chemistry 2024 Nanoscale Open Access Article. Published on 18 September 2024. Downloaded on 9/30/2024 10:30:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online