Facile Synthesis and X-ray Attenuation Properties of Ultrasmall Platinum Nanoparticles Grafted with Three Types of Hydrophilic Polymers

Ultrasmall platinum nanoparticles (Pt-NPs) grafted with three types of hydrophilic and biocompatible polymers, i.e., poly(acrylic acid), poly(acrylic acid-co-maleic acid), and poly(methyl vinyl ether-alt-maleic acid) were synthesized using a one-pot polyol method. Their physicochemical and X-ray attenuation properties were characterized. All polymer-coated Pt-NPs had an average particle diameter (davg) of 2.0 nm. Polymers grafted onto Pt-NP surfaces exhibited excellent colloidal stability (i.e., no precipitation after synthesis for >1.5 years) and low cellular toxicity. The X-ray attenuation power of the polymer-coated Pt-NPs in aqueous media was stronger than that of the commercial iodine contrast agent Ultravist at the same atomic concentration and considerably stronger at the same number density, confirming their potential as computed tomography contrast agents.

Contrast agents are highly valuable materials that facilitate the diagnosis of disease via contrast enhancements [21,22]. Magnetic resonance imaging (MRI) and CT are the most popular imaging modalities for diagnosing diseases, as they provide high-resolution threedimensional anatomical images of the body. The imaging resolution and sensitivity can be further enhanced with contrast agents. Notably, high-contrast images of blood vessels Briefly, three different solutions were prepared, (i) a precursor solution prepared from 1 mmol of Pt precursor in 15 mL of TEG in a 100 mL three-necked round-bottom flask via magnetic stirring at 60 • C under atmospheric conditions (to maintain a constant temperature, the three-necked round-bottom flask was suspended in a silicone oil bath on a hot plate); (ii) an NaOH solution prepared from 9 mmol of NaOH in 20 mL of TEG in a 100 mL beaker via magnetic stirring at 60 • C under atmospheric conditions; and (iii) a polymer solution comprising 0.5 mmol of PAA (or 0.4 mmol of PAAMA or 4.5 × 10 −3 mmol of PMVEMA) in 15 mL of TEG in a 100 mL beaker via magnetic stirring at room temperature under atmospheric conditions. The NaOH solution was slowly added to the Pt-precursor solution with constant magnetic stirring until the pH of the solution reached 9-10. Subsequently, the solution was slowly heated to 110 • C with magnetic stirring, and this temperature was maintained for 4 h. The polymer solution was slowly added to the Pt-precursor solution with magnetic stirring for 12 h, and the NaOH solution was added to maintain the solution pH between 9-10. The product solution was cooled to room temperature and then poured into a 1 L beaker, followed by the addition of 500 mL ethanol and magnetic stirring for 30 min. The diluted product solution was stored in a refrigerator (4 • C) until the polymer-coated Pt-NPs settled at the bottom of the beaker. The top transparent solution was decanted, and the remaining product solution was washed thrice with ethanol using the same process. To remove ethanol from the polymer-coated Pt-NPs, 400 mL of triple-distilled water was added to the product solution, which was subsequently reduced to approximately 50 mL via rotary evaporation. The product solution was further dialyzed using a dialysis tube for one day against 1 L of triple-distilled water with magnetic stirring to remove any remaining impurities; the triple-distilled water was replaced thrice during dialysis. One-half of the sample was dried to powder form for various characterizations, and the remaining half was dispersed in triple-distilled water to prepare an aqueous NP colloidal solution sample.

Synthesis
Ultrasmall Pt-NPs coated with hydrophilic and biocompatible polymers (i.e., PAA, PAAMA, and PMVEMA) were synthesized using a one-pot polyol method ( Figure 1). Briefly, three different solutions were prepared, (i) a precursor solution prepared from 1 mmol of Pt precursor in 15 mL of TEG in a 100 mL three-necked round-bottom flask via magnetic stirring at 60 °C under atmospheric conditions (to maintain a constant temperature, the three-necked round-bottom flask was suspended in a silicone oil bath on a hot plate); (ii) an NaOH solution prepared from 9 mmol of NaOH in 20 mL of TEG in a 100 mL beaker via magnetic stirring at 60 °C under atmospheric conditions; and (iii) a polymer solution comprising 0.5 mmol of PAA (or 0.4 mmol of PAAMA or 4.5 × 10 −3 mmol of PMVEMA) in 15 mL of TEG in a 100 mL beaker via magnetic stirring at room temperature under atmospheric conditions. The NaOH solution was slowly added to the Pt-precursor solution with constant magnetic stirring until the pH of the solution reached 9-10. Subsequently, the solution was slowly heated to 110 °C with magnetic stirring, and this temperature was maintained for 4 h. The polymer solution was slowly added to the Pt-precursor solution with magnetic stirring for 12 h, and the NaOH solution was added to maintain the solution pH between 9-10. The product solution was cooled to room temperature and then poured into a 1 L beaker, followed by the addition of 500 mL ethanol and magnetic stirring for 30 min. The diluted product solution was stored in a refrigerator (4 °C) until the polymer-coated Pt-NPs settled at the bottom of the beaker. The top transparent solution was decanted, and the remaining product solution was washed thrice with ethanol using the same process. To remove ethanol from the polymer-coated Pt-NPs, 400 mL of triple-distilled water was added to the product solution, which was subsequently reduced to approximately 50 mL via rotary evaporation. The product solution was further dialyzed using a dialysis tube for one day against 1 L of triple-distilled water with magnetic stirring to remove any remaining impurities; the triple-distilled water was replaced thrice during dialysis. One-half of the sample was dried to powder form for various characterizations, and the remaining half was dispersed in triple-distilled water to prepare an aqueous NP colloidal solution sample.

General Characterizations
The particle diameters of the polymer-coated ultrasmall Pt-NPs (polymer = PAA, PAAMA, and PMVEMA) were measured using a high-resolution transmission electron microscope (HRTEM) (Titan G2 ChemiSTEM CS Probe, FEI, Hillsboro, OR, USA) operated at an accelerating voltage of 200 kV. For measurements, one drop of diluted NP colloidal sample in triple-distilled water was placed onto a carbon film supported by a 200-mesh copper grid (PELCO no. 160, Ted Pella, Inc., Redding, CA, USA) placed on Whatman filter paper using a micropipette (2-20 mL, Eppendorf, Hamburg, Germany). The copper grid containing the sample was air-dried at room temperature before being loaded into the HRTEM. The scanning transmission electron microscopy (STEM) mode was used to obtain high angle annular dark field (HAADF)-STEM images. An energy-dispersive X-ray spectroscopy (EDS) instrument (Quantax Nano, Bruker, Berlin, Germany) installed inside the HRTEM was used to demonstrate the presence of Pt in the nanoparticle samples.
The crystal structure of the powder samples before and after thermo-gravimetric analysis (TGA) was measured using a powder X-ray diffraction (XRD) spectrometer (X-PERT PRO MRD, Philips, Eindhoven, The Netherlands) with unfiltered CuKa (λ = 1.54184 Å) radiation. The scanning step and scan range in 2θ were 0.03 • and 15-100 • , respectively.
The grafting of the polymers onto ultrasmall Pt-NP surfaces was investigated by recording the Fourier transform-infrared (FT-IR) absorption spectra (Galaxy 7020A, Mattson Instruments, Inc., Madison, WI, USA). For measurements, the powder samples were dried on a hot plate at 40 • C for one week. Pellets of the dried powder samples were prepared in KBr. FT-IR absorption spectra were recorded in the range of 400-4000 cm −1 .
The amount of polymer coating on Pt-NP surfaces was estimated by recording TGA curves using a TGA instrument (SDT-Q600, TA Instruments, New Castle, DE, USA) between room temperature and 700 • C under nitrogen flow. The amount of surface coating (wt. %) was estimated from the mass drop in TGA curves after subtracting the initial mass drop between room temperature and~100 • C owing to water and air desorption.
The Pt concentration in NP colloidal solution samples was measured using an inductively coupled plasma-atomic emission spectrometer (Optima 7300DV, PerkinElmer, Waltham, MA, USA). The colloidal solution samples were pre-treated with acids to completely dissolve the Pt-NPs before measurements. Origin 8.5 (OriginLab, Northampton, MA, USA) was used for data analysis and plot.

In Vitro Cell Viability Assay
The in vitro cellular cytotoxicity of polymer-coated Pt-NPs on the normal human embryonic kidney (HEK293) and human liver tumor (HepG2) cell lines was assessed using a luminescent cell viability assay (CellTiter-Glo, Promega, Madison, WI, USA). The cells were seeded in a 24-well cell culture plate (5 × 10 4 cell density, 500 µL cells/well) and subsequently incubated for 24 h (5% CO 2 , 37 • C). Four test NP colloidal samples (5,10,15, and 20 µM [Pt]) were prepared by diluting concentrated NP colloidal solution samples with sterile phosphate buffer saline solution. Approximately 2 µL of each test NP solution sample was added to the cultured cells, and the cells treated with the test NP solution samples were then incubated for 48 h. Adenosine triphosphate was quantified using a luminometer (Victor 3, PerkinElmer, Waltham, MA, USA), and cell viability was measured thrice and normalized to that of the control cells (i.e., untreated cells with NP samples).

X-ray Phantom Image Measurements
X-ray phantom images were acquired using a micro-CT scanner (Inveon, Simens Healthcare, Erlangen, Germany). Further, X-ray attenuation power was estimated in Hounsfield units (HU) with reference to that of water (0.0 HU) using the formula, HU = 1000 (µ sample − µ water )/µ water , where µ is the linear attenuation coefficient of the materials estimated from the phantom images. The measurements were recorded under the following conditions: X-ray source voltages, 50 and 70 kV p and imaging time per frame, 300 ms.

Physical Characteristics of Polymer-Coated Pt-NPs
The HRTEM images exhibited monodispersed ultrasmall particle size distributions for all polymer-coated Pt-NPs (polymer = PAA, PAAMA, and PMVEMA) (Figure 2a,b). In addition, the HAADF-STEM images (Figure 2c) confirmed monodispersed Pt-NPs in the NP samples. The Pt-elemental mapping over the HAADF-STEM images (Figure 2d) confirmed the presence of uniformly dispersed Pt-NPs in the NP samples. The EDS was also used to confirm the presence of Pt-NPs in the samples by observing transition peaks from Pt (Figure 2e). Fitting the log-normal function to the observed particle diameter distributions suggested that the average particle diameter (d avg ) was approximately 2.0 nm for all polymer-coated Pt-NPs ( Figure 2f and Table 1). Further, fitting the log-normal function to the observed hydrodynamic diameter distributions indicated that the average hydrodynamic diameters (a avg ) were approximately 10.4, 20.5, and 37.5 nm for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively ( Figure 2g and Table 1), showing the proportionality of a avg to the polymer size.
The photographs of NP sample solutions in Figure 3a indicate the lack of NP precipitation after preparing the polymer-coated Pt-NPs in aqueous media (>1.5 years). This highlights their excellent colloidal stability in aqueous media. This finding was also supported by highly negative zeta potentials (−46.5, −44.2, and −57.3 mV for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively; Figure 3b and Table 1). Colloidal dispersion in aqueous media was confirmed by visible laser light scattering (called the Tyndall effect), i.e., laser light scattering was observed only for the vials containing polymer-coated Pt-NP colloidal samples, but not for the vial containing triple-distilled water (Figure 3c).

Physical Characteristics of Polymer-Coated Pt-NPs
The HRTEM images exhibited monodispersed ultrasmall particle size distributions for all polymer-coated Pt-NPs (polymer = PAA, PAAMA, and PMVEMA) (Figure 2a,b). In addition, the HAADF-STEM images ( Figure 2c) confirmed monodispersed Pt-NPs in the NP samples. The Pt-elemental mapping over the HAADF-STEM images (Figure 2d) confirmed the presence of uniformly dispersed Pt-NPs in the NP samples. The EDS was also used to confirm the presence of Pt-NPs in the samples by observing transition peaks from Pt ( Figure 2e). Fitting the log-normal function to the observed particle diameter distributions suggested that the average particle diameter (davg) was approximately 2.0 nm for all polymer-coated Pt-NPs ( Figure 2f and Table 1). Further, fitting the log-normal function to the observed hydrodynamic diameter distributions indicated that the average hydrodynamic diameters (aavg) were approximately 10.4, 20.5, and 37.5 nm for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively ( Figure 2g and Table 1), showing the proportionality of aavg to the polymer size.  The photographs of NP sample solutions in Figure 3a indicate the lack of NP precipitation after preparing the polymer-coated Pt-NPs in aqueous media (>1.5 years). This highlights their excellent colloidal stability in aqueous media. This finding was also supported by highly negative zeta potentials (-46.5, -44.2, and -57.3 mV for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively; Figure 3b and Table 1). Colloidal dispersion in aqueous media was confirmed by visible laser light scattering (called the Tyndall effect), i.e., laser light scattering was observed only for the vials containing polymer-coated Pt-NP colloidal samples, but not for the vial containing triple-distilled water (Figure 3c).
The XRD patterns of all as-prepared powder samples were broad (Figure 4a) owing to ultrasmall particle sizes, as observed in the HRTEM images; all peaks could be assigned with (hkl) Miller indices, i.e., (111), (200), (220), and (311) of the face-centered cubic (fcc) structure of the bulk Pt metal, as labelled on the top of the peaks. Following TGA, all peaks became sharper owing to heat treatment up to 700 • C under nitrogen flow, and in addition, the small (222) peak also appeared ( Figure 4b). This is due to crystal growth during heat treatment [46]. The estimated lattice constant ( ) of 3.911 Å was consistent with the reported value of 3.912 Å [47]. Using the formula of atomic radius r = √ 2 /4 for the fcc structure, r was estimated to be 1.383 Å, which is consistent with the reported value of 1.385 Å [48].
highlights their excellent colloidal stability in aqueous media. This finding was also supported by highly negative zeta potentials (-46.5, -44.2, and -57.3 mV for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively; Figure 3b and Table 1). Colloidal dispersion in aqueous media was confirmed by visible laser light scattering (called the Tyndall effect), i.e., laser light scattering was observed only for the vials containing polymer-coated Pt-NP colloidal samples, but not for the vial containing triple-distilled water (Figure 3c).  The XRD patterns of all as-prepared powder samples were broad (Figure 4a) owing to ultrasmall particle sizes, as observed in the HRTEM images; all peaks could be assigned with (hkl) Miller indices, i.e., (111), (200), (220), and (311) of the face-centered cubic (fcc) structure of the bulk Pt metal, as labelled on the top of the peaks. Following TGA, all peaks became sharper owing to heat treatment up to 700 °C under nitrogen flow, and in addition, the small (222) peak also appeared (Figure 4b). This is due to crystal growth during heat treatment [46]. The estimated lattice constant (ℓ) of 3.911 Å was consistent with the reported value of 3.912 Å [47]. Using the formula of atomic radius r = √2ℓ/4 for the fcc structure, r was estimated to be 1.383 Å, which is consistent with the reported value of 1.385 Å [48].

Polymer-Coating Amount and Structure
The polymer coating (polymer = PAA, PAAMA, and PMVEMA) on Pt-NP surfaces was investigated by recording the FT-IR absorption spectra and TGA curves. The characteristic vibrational absorption frequencies of the polymers were observed at ~2930 (C-H symmetric stretch), ~1695 (C=O symmetric stretch), ~1555 (COOantisymmetric stretch), and ~1390 cm -1 (COOsymmetric stretch) in the FT-IR absorption spectra of polymercoated Pt-NP samples, confirming successful polymer coating on the Pt-NP surfaces (Figure 5a-c). The observed absorption frequencies are listed in Table 2. The C=O symmetric stretch at ~1695 cm -1 was reduced in intensity and was also red-shifted and split into symmetric and antisymmetric stretches of COOafter coating; these could be attributed to the electrostatic bonding between COOgroups of the polymers and Pt atoms on the NP surfaces [49,50]. This corresponds to a hard acid (Pt atoms on the NP surfaces) hard base (COOgroups of the polymers) type of bonding [51], as depicted in Figure 5d. Owing to the numerous COOgroups per polymer, multiple bonds can exist between many COO − groups of a polymer and many Pt atoms on a Pt-NP surface.

Polymer-Coating Amount and Structure
The polymer coating (polymer = PAA, PAAMA, and PMVEMA) on Pt-NP surfaces was investigated by recording the FT-IR absorption spectra and TGA curves. The characteristic vibrational absorption frequencies of the polymers were observed at~2930 (C-H symmetric stretch),~1695 (C=O symmetric stretch),~1555 (COO − antisymmetric stretch), and 1390 cm −1 (COO − symmetric stretch) in the FT-IR absorption spectra of polymer-coated Pt-NP samples, confirming successful polymer coating on the Pt-NP surfaces (Figure 5a-c). The observed absorption frequencies are listed in Table 2. The C=O symmetric stretch at 1695 cm −1 was reduced in intensity and was also red-shifted and split into symmetric and antisymmetric stretches of COO − after coating; these could be attributed to the electrostatic bonding between COO − groups of the polymers and Pt atoms on the NP surfaces [49,50]. This corresponds to a hard acid (Pt atoms on the NP surfaces) hard base (COO − groups of the polymers) type of bonding [51], as depicted in Figure 5d. Owing to the numerous COO − groups per polymer, multiple bonds can exist between many COO − groups of a polymer and many Pt atoms on a Pt-NP surface.  The amount (P) of polymer coating on the Pt-NP surfaces in wt. %, ranging from 34 to 55%, was obtained from TGA curves by measuring mass losses after considering initial mass drops between room temperature and ~100 °C owing to water and air desorption from powder samples ( Figure 6 and Table 3). The remaining mass drops were attributed to the removal of polymers from the Pt-NPs during heating to 700 °C under nitrogen flow. Grafting densities (σ), corresponding to the average number of polymers coating a NP unit surface area [52,53], were estimated to be 1.2, 1.0, and 0.1 nm −2 for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively, using the Pt bulk density (21.45 g/cm 3 ) [54], the davg values estimated from HRTEM imaging, and the p values obtained from the TGA curves. The average number (Npolymer) of polymers coating an NP surface was estimated from the product, σ × NP surface area, in which the NP surface area was approximately estimated from πdavg 2 . The coating results are summarized in Table 3. Under these coating  The amount (P) of polymer coating on the Pt-NP surfaces in wt. %, ranging from 34 to 55%, was obtained from TGA curves by measuring mass losses after considering initial mass drops between room temperature and~100 • C owing to water and air desorption from powder samples ( Figure 6 and Table 3). The remaining mass drops were attributed to the removal of polymers from the Pt-NPs during heating to 700 • C under nitrogen flow. Grafting densities (σ), corresponding to the average number of polymers coating a NP unit surface area [52,53], were estimated to be 1.2, 1.0, and 0.1 nm −2 for PAA-, PAAMA-, and PMVEMA-coated Pt-NPs, respectively, using the Pt bulk density (21.45 g/cm 3 ) [54], the d avg values estimated from HRTEM imaging, and the p values obtained from the TGA curves. The average number (N polymer ) of polymers coating an NP surface was estimated from the product, σ × NP surface area, in which the NP surface area was approximately estimated from πd avg 2 . The coating results are summarized in Table 3. Under these coating conditions, excellent colloidal stability was observed in aqueous media, indicating sufficient coating.

In Vitro Cellular Cytotoxicity Results
The cellular cytotoxicity of polymer-coated ultrasmall Pt-NP samples was assessed by measuring the viability of HepG2 and HEK293 cells 48 h after incubation with the NP samples. As shown in Figure 7, cell viability was high (>75%) up to the tested concentration range (20 µM [Pt]), confirming the low toxicity of all polymer-coated ultrasmall Pt-NPs.

In Vitro Cellular Cytotoxicity Results
The cellular cytotoxicity of polymer-coated ultrasmall Pt-NP samples was assessed by measuring the viability of HepG2 and HEK293 cells 48 h after incubation with the NP samples. As shown in Figure 7, cell viability was high (>75%) up to the tested concentration range (20 µM [Pt]), confirming the low toxicity of all polymer-coated ultrasmall Pt-NPs. conditions, excellent colloidal stability was observed in aqueous media, indicating sufficient coating.

In Vitro Cellular Cytotoxicity Results
The cellular cytotoxicity of polymer-coated ultrasmall Pt-NP samples was assessed by measuring the viability of HepG2 and HEK293 cells 48 h after incubation with the NP samples. As shown in Figure 7, cell viability was high (>75%) up to the tested concentration range (20 µM [Pt]), confirming the low toxicity of all polymer-coated ultrasmall Pt-NPs.

X-ray Phantom Images and X-ray Attenuation Power
The X-ray attenuation properties of PAA-, PAAMA-, and PMVEMA-coated Pt-NPs in aqueous media were characterized by capturing X-ray phantom images at 50 and 70 kV p ( Figure 8). The results were compared with those of a commercial iodine contrast agent, Ultravist. The X-ray phantom images of the sample solutions were slightly brighter than those of Ultravist at a similar atomic concentration (Figure 8) because of the stronger X-ray attenuation coefficient of Pt compared with that of iodine [17]. In addition, the phantom images became slightly brighter with increasing X-ray source voltages from 50 to 70 kV p ; this is owing to a wider X-ray energy range coverage at 70 kV p and under such conditions, the X-ray attenuation coefficient becomes enhanced, thus providing brighter X-ray phantom images. To quantitatively investigate X-ray attenuation properties, Xray attenuation power was estimated from the X-ray phantom images. As shown in Figure 9a, the X-ray attenuation power of the sample solutions was stronger than that of the Ultravist at the same atomic concentration. In addition, the X-ray attenuation power of the sample solutions and Ultravist was plotted as a function of the number density to further confirm the superiority of polymer-coated Pt-NPs to Ultravist (Figure 9b). To plot the X-ray attenuation power as a function of number density, the number density was estimated as the molar atomic concentration × 6.02 × 10 23 ÷ N atom , where N atom is the number of atoms per molecule or NP, which is three for Ultravist and~(d avg /2r) 3 [=(2.0/0.2766) 3 = 378] for Pt-NPs (Table 4), in which 2r is the Pt atomic diameter of 0.2766 nm estimated from the lattice constant, as described before, and d avg is the average NP diameter of 2.0 nm (Table 1) obtained from HRTEM imaging. As plotted in Figure 9b, the X-ray attenuation power of the sample solutions was significantly higher than that of Ultravist at the same number density. the X-ray attenuation power of the sample solutions was stronger than that of the Ultr at the same atomic concentration. In addition, the X-ray attenuation power of the sam solutions and Ultravist was plotted as a function of the number density to further con the superiority of polymer-coated Pt-NPs to Ultravist (Figure 9b). To plot the X-ray a uation power as a function of number density, the number density was estimated a molar atomic concentration × 6.02×10 23 ÷ Natom, where Natom is the number of atoms molecule or NP, which is three for Ultravist and ~(davg/2r) 3 [= (2.0/0.2766) 3 = 378] fo NPs (Table 4), in which 2r is the Pt atomic diameter of 0.2766 nm estimated from the la constant, as described before, and davg is the average NP diameter of 2.0 nm (Table 1 tained from HRTEM imaging. As plotted in Figure 9b, the X-ray attenuation power o sample solutions was significantly higher than that of Ultravist at the same number sity. The X-ray attenuation efficiency (η), which corresponds to the molar X-ray atte tion power or X-ray attenuation power per number density, is a primary paramete comparing the X-ray attenuation power of the materials. As a function of atomic con tration (slopes in Figure 9a), the η values of the Ultravist and sample solutions were mated to be 4.0 and 16.4 HU/mM at 50 kVp, respectively, and 5.0 and 18.4 HU/mM kVp, respectively ( Table 4). As a function of number density (slopes in Figure 9b), were estimated to be 0.20 × 10 -20 and 102.7 × 10 -19 HU/(1/L) at 50 kVp, respectively, and × 10 -20 and 115.1 × 10 -19 HU/(1/L) at 70 kVp, respectively (Table 4). Thus, the Pt-NP sam exhibited approximately four times and 500 times higher X-ray attenuation power Ultravist at the same atomic concentration and number density, respectively. The η va are plotted in Figure 9c, clearly showing the higher η values of the Pt-NP samples. T results confirm the superiority of polymer-coated Pt-NPs to iodine contrast agents. The X-ray attenuation efficiency (η), which corresponds to the molar X-ray attenuation power or X-ray attenuation power per number density, is a primary parameter for comparing the X-ray attenuation power of the materials. As a function of atomic concentration (slopes in Figure 9a), the η values of the Ultravist and sample solutions were estimated to be 4.0 and 16.4 HU/mM at 50 kV p , respectively, and 5.0 and 18.4 HU/mM at 70 kV p , respectively ( Table 4). As a function of number density (slopes in Figure 9b), they were estimated to be 0.20 × 10 −20 and 102.7 × 10 −19 HU/(1/L) at 50 kV p , respectively, and 0.25 × 10 −20 and 115.1 × 10 −19 HU/(1/L) at 70 kV p , respectively (Table 4). Thus, the Pt-NP samples exhibited approximately four times and 500 times higher X-ray attenuation power than Ultravist at the same atomic concentration and number density, respectively. The η values are plotted in Figure 9c

Discussion
PAA-, PAAMA-, and PMVEMA-coated ultrasmall Pt-NPs were synthesized using a simple one-pot polyol method. This method is considerably simpler compared with previously reported methods [16,[32][33][34][35][36]. All polymer-coated Pt-NPs were stable in colloidal form in aqueous media and nearly non-toxic. As displayed in Figure 1, all of the polymers used contain numerous hydrophilic -COOH groups; consequently, they can strongly bind to the Pt-NPs and attract numerous water molecules around the NPs, thereby providing large hydrodynamic diameters and excellent colloidal stability. In addition, the hydrodynamic diameter increased with increasing polymer size from PAA → PAAMA → PMVEMA. This is because larger polymers are bulkier and contain more hydrophilic -COOH groups that can attract water molecules around the Pt-NPs, thus providing larger hydrodynamic diameters.
The stronger X-ray attenuation power of the sample solutions (compared to that of Ultravist) was indicated by the brighter phantom images of the sample solutions at similar atomic concentration (Figure 8). We quantitatively validated this by estimating the X-ray attenuation power values from the phantom images and plotting them as a function of atomic concentration [Pt] or [I] (Figure 9a and Table 4). Furthermore, the X-ray attenuation power of the sample solutions was significantly higher than that of Ultravist at the same number density (Figure 9b). This result is important because the osmolality and viscosity of the contrast agents mainly depend on the number density rather than the atomic concentration [23]. Contrast agents with high osmolality and viscosity can cause side effects in patients with kidney injury [55]. However, NP contrast agents can have lower viscosity and osmolality than iodine contrast agents owing to their lower number density at the same atomic concentration [23]. In addition, the injection dose of Pt-NP contrast agents can be further reduced because of the higher X-ray attenuation coefficient of Pt compared with that of iodine [17].
The X-ray attenuation efficiency (η) is a primary parameter indicating the performance of X-ray contrast agents. Our results were compared with those of other studies [16,[32][33][34][35][36]. As shown in Table 5, our Pt-NP η value was higher compared with other values at the same X-ray source voltage (kV p ) because the η value increases with X-ray source voltage. This higher value can be explained as follows. The η value decreases with particle size but increases with colloidal stability. For NPs with poor colloidal stability, the precipitated NPs poorly contribute to X-ray attenuation, resulting in low η values. Our hydrophilic polymercoated ultrasmall Pt-NPs exhibited excellent colloidal stability (i.e., no precipitation after synthesis), thus rendering a high η value. The internal Pt atoms in larger NPs contribute less to the X-ray attenuation compared with surface Pt atoms because of the penetration depth limit of X-rays in metals. As shown in Table 5, our Pt-NPs have smaller particle diameters compared with those of other Pt-NPs, explaining the obtained higher η value compared with others. Thus, given all of the above findings, the hydrophilic polymercoated ultrasmall Pt-NPs (polymer = PAA, PAAMA, and PMVEMA) synthesized in this study constitute promising potential high-performance X-ray contrast agents.

Conclusions
In summary, we used for the first time a one-pot polyol method to synthesize ultrasmall Pt-NPs coated with three types of hydrophilic and biocompatible polymers (i.e., PAA, PAAMA, and PMVEMA). We evaluated their cytotoxicity using HepG2 and HEK293 cell lines, and characterized their physicochemical and X-ray attenuation properties. 1.
The observed average particle diameter was nearly monodispersed and ultrasmall (i.e., 2.0 nm) for all polymer-coated Pt-NPs;

2.
Highly negative zeta potentials (<−40 mV) were observed for all polymer-coated Pt-NP solution samples owing to the coating of hydrophilic and biocompatible polymers on the NP surfaces. This led to excellent colloidal stability (no precipitation after synthesis for >1.5 years). Furthermore, all polymer-coated Pt-NP solution samples exhibited low toxicity (>75% cell survival) up to the tested concentration range of 20 µM [Pt], indicating their suitability for biomedical applications; 3.
The X-ray attenuation power of all polymer-coated Pt-NP solution samples was 4 times higher than that of the commercial iodine contrast agent Ultravist at the same atomic concentration and~500 times higher at the same number density, confirming the superiority of the polymer-coated Pt-NPs to iodine contrast agents and thus, their potential as viable high-performance CT contrast agents.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.