Adsorption of Phosphate and Polyphosphate on Nanoceria Probed by

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Adsorption of Phosphate and Polyphosphate on Nanoceria Probed by DNA Oligonucleotides Xiuzhong Wang, Anand Lopez, and Juewen Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01482 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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Adsorption of Phosphate and Polyphosphate on Nanoceria Probed by DNA Oligonucleotides

Xiuzhong Wang,1,2 Anand Lopez2 and Juewen Liu2*

1. College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China

2. Department of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Email: [email protected]

1

ACS Paragon Plus Environment

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Abstract Phosphate containing molecules exist in many forms in biology and environment, and their interaction with metal oxides is an important aspect of their chemistry and biochemistry. In this work, phosphates with different degrees of polymerization (e.g. orthophosphate, pyrophosphate, sodium triphosphate (STPP), trimetaphosphate (STMP), and polyphosphate with 25 phosphate units) and phosphates with one or two capping groups were studied. CeO2 nanoparticles (nanoceria) were used as a model metal oxide. DNA is also a polyphosphate, and a fluorescently labeled DNA oligonucleotide was mixed with nanoceria. These phosphate species were individually added to displace the adsorbed DNA. Longer phosphate chains were more efficient when each molecule was used at the same molar concentration, while pyrophosphate and STPP were most efficient at the same total phosphorus atom concentration. By capping the phosphate with organic groups, the affinity was significantly decreased. Isothermal titration calorimetry (ITC) was also performed to quantitatively measure thermodynamic parameters. While STMP was very slow at displacing DNA, it was still adsorbed very strongly by nanoceria from ITC, indicating kinetic effects likely due to its ring structure. This observation allowed us to use the DNA as a probe to study the hydrolysis of STMP to form STPP. In summary, this study provides a systematic understanding of phosphate species interacting with metal oxides and it demonstrates interesting an analytical application as well.

Keywords: metal oxides; biosensors; desorption; fluorescence; quench 2


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Introduction Phosphate is a key building block in biology.1,

2

Nucleic acids have a phosphate

backbone, while most lipids contain a bridging phosphate. Even though phosphate is not present in the natural amino acids, phosphorylated proteins are key for cell signaling. Unlike the bridging phosphates in nucleic acids and lipids, the phosphates in proteins are terminal. Aside from these biomacromolecules, phosphate also exists in many small molecules. The most important energy molecule, adenosine triphosphate (ATP), contains a linear triphosphate. Its hydrolysis to AMP releases a pyrophosphate (PPi). Phosphate can also polymerize forming dimers, trimers, polymers and cyclized structures.3, 4 Finally, the oxygen atoms in phosphate can also be capped by various ligands. Due to its chemical inertness, however, phosphate is often overlooked. Phosphate has a high adsorption affinity for many metal oxides, and this property has resulted in some interesting applications. For example, TiO2 was commonly used to extract phosphorylated proteins for subsequent analysis.5 The phosphate group in lipids was believed to be important for adsorption to some metal oxide surfaces, and engineered lipids with exposed phosphate were found to be even better for this purpose.6, 7 DNA oligonucleotides can be stably adsorbed by many metal oxides,8 such as CeO2,9 Fe3O4,10,

11

ZnO,12-14 and TiO2.15,

16

In each case, the phosphate

backbone of DNA played the main role for adsorption, and inorganic phosphate can displace the adsorbed DNA. Interactions between phosphate and minerals are also important.

For example,

some

RNAs such

as poly(ADP-ribose)

contain 3


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pyrophosphate linkages,17 and they are believed to be a key factor for organizing cellular architectures and biomineralization.18 Calcium phosphate is the key building block of the hard tissues.19, 20 Given the complexity of phosphate chemistry, it is important to have a systematic understanding of their adsorption on metal oxides. Such understandings are useful for biosensor development,21 biomineralization, regulating catalytic activity of metal oxides (called nanozymes),22, 23 and environmental remediation.24-26 Taking advantage of DNA being a polyphosphate and the fluorescence quenching property of many materials,27-30 we reason it might be possible to use DNA as a probe to understand the adsorption of various phosphates.28, 31, 32 In this work, we performed a comprehensive study of the interaction between various phosphate species and a model metal oxide, cerium dioxide (CeO2). Quantitative measurements using isothermal titration calorimetry (ITC) were also carried out. Related findings allowed us to further develop an assay for an important phosphate reaction.

Materials and Methods Chemicals. All the DNA samples were purchased from Integrated DNA Technologies (IDT, Coralville, IA, USA), and were purified by standard desalting. The sequence of the FAM-labeled 24-mer DNA is FAM-ACGCATCTGTGAAGAGAACCTGGG, with the FAM on the 5′-end. Sodium chloride, sodium hydroxide, and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were purchased from Mandel Scientific (Guelph, Ontario, Canada). CeO2 NPs, Malachite green, sodium 4


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molybdate

dihydrate,

sodium

pyrophosphate

decahydrate

(PPi),

sodium

polyphosphate (Pi25), sodium trimetaphosphate (STMP), sodium triphosphate (STPP) and dimethyl phosphate (DMP) were purchased from Sigma-Aldrich. Choline dihydrogen phosphate was purchased from Ionic Liquid Technologies. Milli-Q water was used to prepare all the buffers and solutions. DNA-based probing. To study the DNA adsorption and desorption by CeO2, 50 nM FAM-labeled DNA was dissolved in HEPES buffer (10 mM, pH 7.4, with 200 mM NaCl). To optimize CeO2, different concentrations of CeO2 NPs were added to the above solution and incubated for 30 min before the fluorescence was recorded (Eclipse, Varian). The desorption kinetics were recorded after a quick addition of phosphate to the FAM-DNA/CeO2 mixture (final concentration = 5 µg/mL CeO2) in the above HEPES buffer followed by fluorescence recording for 90 min at room temperature (~23 °C). The fluorescence was compared with the initial intensity of the free DNA to calculate the adsorbed DNA. This buffer and CeO2 concentration were for all the experiment unless otherwise indicated. ITC. All calorimetric titrations were performed with a microcalorimeter instrument (MicroCal) at 25 °C. All phosphates were prepared using HEPES buffer (10 mM, pH 7.4, 200 mM NaCl). The CeO2 concentration in the cell was 100 µg/mL (1.4551 mL in the ITC parameter setting), and the phosphate concentration in the injection syringe was 1.0 mM. Each titration consisted of an initial injection of 1 µL followed by 28 injections of 10 µL spaced 300 sec apart. The initial 1 µL injection was affected by diffusion into and out of the injection syringe during the initial equilibration period. 5


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Data from this injection were omitted from the analysis. Background heat of buffer injections into CeO2 solution was subtracted from the corresponding titrations. The enthalpy (∆H) and binding constant (Ka) were obtained through fitting the titration curves to a one-site binding model using the Origin software. ∆S was calculated from ∆G = ∆H − T∆S. Colorimetric quantification of phosphate. The malachite green colorimetric assay procedures were used according to the literature with minor modifications.33 The malachite green reaction solution (R1) was prepared as follows. First, 0.04 g sodium molybdate dihydrate was dissolved in 5 mL of HCl (4 M). Then, this solution was mixed with 15 mL of 0.045% (w/v) malachite green aqueous solution and stirred for 30 min until it became clear. After centrifugation, the supernatant was stored at 4 °C for use. Eight microliters of various concentrations of phosphate was placed into microcentrifuge tubes and mixed with 8 µL of 0.1 M HCl, 104 µL distilled water and 80 µL R1 for 10 min. A mixture of the respective phosphate concentrations with distilled water was prepared as controls. A mixture of R1 (80 µL), the respective acidic solution (8 µL) and distilled water (112 µL) was used as blank for UV-vis spectroscopy. The absorbance was read at 670 nm in a UV spectrometer (Agilent 8453A) at room temperature. The experiments were run with three repeats in series. Hydrolysis of STMP. 10 mM STMP and 10 mM STPP were respectively incubated in 2.0, 1.0, 0.5 and 0.1 M NaOH for 60 min, after which the solutions were diluted over 1000-fold in buffer to ensure a neutralized pH. The samples were then used for desorbing DNA from CeO2 as described above. The final STMP (if no hydrolysis 6


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occurred) was 10 µM.

Results and Discussion Phosphate species studied. In this work, we studied a total of eight phosphate species with different numbers of phosphate unit or capping groups (Figure 1). For various degrees of polymerization, we included simple inorganic orthophosphate (Pi), pyrophosphate (PPi), triphosphate (STPP), polyphosphate with 25 phosphate units (Pi25), and trimetaphosphate (STMP) (Figure 1A). Among them, STMP is cyclic, while the rest are linear. The second group was based on Pi (Figure 1B), and we capped it by one (phosphocholine, PC) or two groups (dimethyl phosphate, DMP).

Figure 1. The structure of phosphate containing species studied in this work: (A) phosphates of different degrees of polymerization; (B) capping inorganic phosphate 7


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with one or two groups. (C) The structure of a 4-mer DNA oligonucleotide with a polyphosphate backbone (this study used a longer DNA and this is only for showing the structure).

These species are representative in terms of phosphate chemistry and allow a systematic study. Note that at neutral pH, a fraction of the phosphate groups were protonated, depending on the pKa of each group, but these protons were not drawn in Figure 1 for clarity. The structure of a 4-mer DNA oligonucleotide is also shown (Figure 1C). DNA is a polyphosphate and all of its phosphate groups are in the bridging form with two of the oxygen atoms capped, which is similar to DMP. Fluorescent DNA as a probe. DNA is a polyphosphate, and it can strongly adsorb on various metal oxides mainly via its phosphate backbone.12,13 The affinity of adsorption can be readily tuned by varying the length of DNA, while DNA sequence is often less important.10,

12

For example, cerium oxide (CeO2) nanoparticles

(nanoceria) strongly adsorb DNA and also quench fluorescence. Previous studies confirmed the importance of DNA phosphate backbone for the adsorption.9, 34 CeO2 is also highly important for its catalytic activities,35 and it has been tested for analytical,36 and biomedical applications.37-39 For these reasons, we chose CeO2 in this work as a model oxide. Our CeO2 nanoparticles were ~5 nm (Figure 2A for TEM), and adding increasing concentrations of CeO2 to a carboxyfluorescein (FAM) labeled DNA gradually quenched the fluorescence (Figure 2B). Even 10 µg/mL of CeO2 fully quenched the emission from 50 nM of a 24-mer DNA. This DNA was chosen as a 8


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representative random sequence. As explained above, the effect of DNA sequence should be small.9 We reason that various phosphorus containing species might compete with the DNA and displace it from the particle surface (Figure 2C). A stronger adsorbing species may induce faster DNA desorption and thus faster fluorescence enhancement. This method may allow us to conveniently study adsorption of these phosphate species.

Figure 2. (A) A TEM micrograph of the CeO2 nanoparticles used in this work. (B) Quenching of a FAM-labeled 24-mer DNA with a random sequence (50 nM) by different concentrations of CeO2 nanoparticles in buffer (10 mM HEPES, pH 7.45, 200 mM NaCl). (C) A scheme showing adsorbed DNA as a probe to study adsorption of phosphate species.

Phosphate with different degrees of polymerization. With this DNA-based probe, we first tested the phosphate species in Figure 1A. When 10 µM of each phosphate species was respectively added to the FAM-DNA/CeO2 complex, we observed very different kinetics of fluorescence enhancement. The highest enhancement was 9


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observed with Pi25, followed by STPP, PPi and phosphate (Figure 3A). This can be easily explained by the length of the phosphate chain. The longer the phosphate chain, the stronger the adsorption affinity. The only outlier was STMP, which was the slowest among these species despite the fact that it contains a total of three phosphate groups. Since different compounds contained different number of phosphate units, to make a fair comparison, we then prepared another batch of samples with the same concentration of the total phosphorus atom of 10 µM (Figure 3B). In this case, the PPi and STPP samples had a similar high response. Out of the three phosphate groups in STPP, two are terminal phosphates and one is bridged in the middle. We reason that the bridging phosphate might contribute less to adsorption. A previous work on Pi and PPi adsorption by iron oxide surface has indicated that the number of Fe-O-P linkages is important for the final binding strength,40 and other detailed studies have also been reported at such levels.41, 42 Terminal phosphate can establish more linkages with the surface and thus has a higher affinity. PPi has only terminal phosphates and it showed strong binding even though it should be less efficient in terms of polyvalent binding.43 On the other hand, the molar concentration of PPi was 1.5-fold that of STPP, and this may also contribute to the higher signal. By comparing Pi and PPi, we can see that polyvalency contributed significantly. Despite the molar concentration of PPi being only half of Pi, PPi had doubled signal, and this can only be attributed to the polvalency effect. The Pi25 sample was even slower than that of the Pi sample, and this can be rationalized by 10


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most of its phosphates being bridging ones. Since its molar concentration was 25-fold lower, the disadvantage of the bridging phosphate became very obvious. It is quite interesting to notice that the cyclic STMP had a very slow response, although it also has three phosphate units. For STMP, all of its phosphates are bridging. The shape of its kinetic curve was also different (Figure 3A). Initially its kinetics was very slow followed by a faster phase, and the overall trend appeared to have an induction period. STMP might have a hard time competing with DNA kinetically. Only when adsorbed at a sufficiently high concentration, can it compete with the DNA. The cyclic geometry of STMP may be the key reason for its kinetic disadvantage.

Figure 3. Kinetics of fluorescence enhancement indicative of FAM-labeled DNA displaced by the added phosphate species: (A) 10 µM of each species; and (B) 10 µM of total phosphate unit. Different concentrations of (C) PPi and (D) STMP were added to induce desorption of the DNA. 11


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To further understand their adsorption, we studied the effect of phosphate concentration. A typical first-order kinetics was observed for PPi (Figure 3C), whereas STMP at below 100 µM showed a sigmoidal shape (Figure 3D). The PPi sample achieved half of fluorescence increase with just 5.0 µM, while over 200 µM STMP was needed to induce the same increase in 90 min. The PPi sample reached equilibrium quickly within 10 min even with sub-µM PPi concentrations, while STMP displaced the DNA much slower, and equilibrium was not reached for most samples even after 3 h. Note that after a long incubation, the STMP signal catched up gradually. Therefore, PPi displaced the DNA much faster, while the adsorption affinity was hard to judge by this DNA probe. ITC analysis. To quantitatively understand the adsorption of these phosphate species, we then performed isothermal titration calorimetry (ITC) by gradually titrating each phosphate sample into a dispersion of CeO2 (no DNA added), and the rate of heat release was recorded (Figure 4A-F, top panels). ITC is a powerful biophysical technique that allows quantitative measurement of binding reactions, including molecular adsorption on surfaces.44-47 All these reactions were initially exothermic as indicated by the downward spikes of the thermograms. With increasing length of the phosphate chain, lesser injections were required to reach saturation. In particular, the Pi25 sample saturated after just two injections (Figure 4E). This was explained by the limited surface area of CeO2, and the phosphate chains were adsorbed lengthwise. We then integrated the heat (bottom panels) and fitted it with a single binding site model. 12


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To have an accurate fitting, we need to find the number of adsorption sites on each CeO2 nanoparticle and we used a colorimetric assay for this purpose. Malachite green forms a strongly absorbing species at around 670 nm (molar absorption coefficient, 90,000 M-1cm-1) after complexing with phosphomolybdate and changes its color from light green to deep blue.33 The inset of Figure 4G shows the color of the samples containing molybdate, malachite green and different concentrations of Pi. A nice color gradient was observed. These samples were used to build a calibration curve for subsequent quantification of Pi adsorption. We then added Pi of different concentrations to a fixed concentration of CeO2, and the samples were centrifuged to precipitate CeO2 with adsorbed Pi. Then, the non-adsorbed Pi was measured to construct an adsorption isotherm (Figure 4G). From this, we fitted a Langmuir adsorption model and obtained the saturated adsorption capacity. Taking the average size of each CeO2 nanoparticle to be 5 nm, we calculated its molar concentration to be 0.69 µM for a 100 mg/mL dispersion. Therefore, each CeO2 nanoparticle adsorbed 160 Pi ions. With the number of adsorption site determined, we quantitatively fitted the data and obtained all the thermodynamic parameters (Table 1). The binding ratio (N) between phosphate and CeO2 decreased with increasing length of the phosphate chain. Pi has a ratio of ~107, and this is similar to the 160 determined from the colorimetric method. The difference could be attributed to the two different techniques used, including possible insufficient separation of CeO2 in the colorimetric method (e.g. centrifugation at 15,000 rpm may not fully remove all the CeO2 nanoparticles). PPi 13


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had the ratio around 60 which was about half that of Pi, suggesting that it can also fully occupy the surface of CeO2. STPP and STMP were adsorbed at around 30 molecules on each CeO2. Multiplying this number by three reached around 90 phosphate groups and this number was also close to 107 for Pi adsorption. Therefore, it is likely that all these species can almost fully cover the surface adsorption sites. The only exception was the Pi25 chain, which saturated after just the first injection. It had a much lower total phosphate unit of just 20 (not shown). This number is unlikely to be accurate since only one main peak was available, and we could not obtain an accurate fitting with this data. We then decreased the concentration of Pi25 by 10-fold and performed a new titration. In this case, although the number of peaks increased, the data did not follow a simple trend (Figure 4F). We still fitted it to the same model and obtained a binding ratio of around 50 phosphate unit. We attributed the lower number of adsorbed phosphate here to its polymeric nature, which may crosslink the CeO2 nanoparticles to mask some adsorption sites. While the initial injections all released heat, later injections showed heat absorption for many samples. It appears that the longer the phosphate chain, the more heat was absorbed for the later injections (e.g. comparing Figure 4A, B, C, F). Heat absorption is indicative of entropy-driven reactions. Adsorption of a molecule is typically accompanied with entropy loss, while an increase of entropy is often associated with release of structured water molecules. Since our entropy change was proportional to the phosphate chain length, it was likely from the release of water molecules associated with the bridging phosphates. 14


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We then compared the association constants (Ka), which is a measurement of adsorption affinity. Among all the species, PPi had the highest adsorption affinity followed by STTP. This is consistent with their excellent ability to displace DNA. Although the affinity of STMP was slightly weaker than PPi and STTP, it was 4-fold stronger than that of Pi. On the other hand, STMP was not as effective as Pi for displacing the DNA. Therefore, STMP was indeed kinetically disadvantageous in the presence of pre-adsorbed DNA. With a free CeO2 surface, its adsorption was quite strong. This is an interesting observation and it might be related to its ring structure, which makes it more difficult to quickly achieve stable adsorption in the presence of a competing DNA.

Figure 4. ITC traces and integrated heat with fitting of one-site binding model for titrating (A) PO43-, 1.0 mM; (B) PPi, 1.0 mM; (C) STPP, 0.5 mM; (D) STMP, 0.5 mM; 15


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(E) Pi25, 1 mM; and (F) Pi25, 0.1 mM into CeO2 nanoparticles. (G) Pi adsorption isotherm by CeO2 determined using the malachite green colorimetric method. The trend was fitted to a Langmuir isotherm. Inset: a photograph showing the color of these samples with 20, 50, 100, 200, 400, 800 and 1200 µM Pi.

The enthalpy of the reactions increased gradually with the chain length of the phosphate species, indicating that most phosphate units in these molecules contributed to adsorption. The fitting was mainly based on the first a few titrations, from which the entropy was all negative. A longer phosphate chain had a higher initial entropy value and after adsorption its entropy also decreased more. This is consistent with a typical binding reaction of hydrophilic species in water. As discussed above, a second stage of binding driven by entropy could also exist at higher phosphate concentrations.

Table 1. Thermodynamic parameters from ITC fitting. n

Ka (× 105 M−1)

∆H (kcal mol−1)

∆S (cal K-1 mol-1)

Pi-

95.0 ± 8.0

1.33

-7.190 ± 0.34

-0.674

PPi

60.0 ± 2.0

33.4

-9.607 ± 0.33

-2.37

STPP

21.4 ± 3.7

16.6

-10.79 ± 1.82

-18.5

STMP

26.8 ± 0.2

49.6

-22.27 ± 0.25

- 44.1

Pi25

2.0 ± 0.42

97.8

-79.21 ± 12.5

-234

PC

90.7 ± 3.44

0.328

-7.535 ± 0.11

-4.61 16


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DMP

−

−

−

−

Monitoring of STMP hydrolysis to form STPP. The very different DNA desorption kinetics in the presence of STMP and STPP may allow us to use this system to monitor the hydrolysis of STMP. Typically, this reaction needs to be monitored by mass spectrometry, chromatography or NMR.48, 49 A simpler biosensor method could allow more researchers to study this important reaction, which is related to chemical synthesis of ATP and other phosphorylated compounds.50 We incubated STMP in NaOH solutions of different concentrations (and thus different pH) for 60 min and then the mixture was diluted ~1000-fold in buffer to ensure neutralized pH. These samples were then added to the FAM-DNA/CeO2 mixture (Figure 5A). With increasing NaOH concentrations, higher fluorescence enhancement was observed. This can be explained by the faster hydrolysis of STMP in basic solutions. We also compared the response of the same mixture of STPP mixed in the same base solutions (Figure 5B). In this control case, all the samples showed a similar response. The change observed in Figure 5A was indeed due to the hydrolysis reaction. Therefore, this simple DNA-based assay system could be used to monitor this difficult reaction.

17


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Figure 5. Kinetics of fluorescence enhancement of the FAM-DNA/CeO2 complex mixed with neutralized (A) STMP; and (B) STTP solutions pre-incubated with various concentrations of NaOH for 60 min. The initial STMP and STTP concentrations were 10 mM, and in the final mixture, their concentrations were 10 µM.

Phosphate with capping groups. The above studies all focused on polyphosphates. Another important aspect is to cap the simple inorganic phosphate with different groups. This is often seen in biological molecules. For example, the phosphate group in phospholipids have two capping groups and so is the phosphate in nucleic acids. Many small molecules such as phosphocholine (PC), AMP and FMN, have one capping group. In this study, we compared three molecules as shown Figure 1B to understand the capping effect. The same DNA-based method was used to study the displacement reaction (Figure 6A). Interestingly, Pi and PC had a very similar response, and thus a single capping group did not seem to affect the affinity to CeO2. This can probably be 18


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explained by the geometry of the molecule. With a tetrahedral shape, not all the four oxygen atoms in the Pi can bind to the surface at the same time. On the other hand, dimethyl phosphate (DMP) had almost no response, demonstrating that two capping groups have significantly decreased binding affinity. To further study this system, we also performed ITC (Figure 6B-D). Indeed, the adsorption profiles were quite similar for Pi and PC in terms of adsorption capacity (Table 1, last entries), while almost no heat was produced from the DMP sample. Quantitative fitting also showed that Pi had around 4-fold higher adsorption affinity (Ka) compared to PC, and thus capping even one oxygen was still detrimental for the adsorption affinity. Interestingly, Pi and PC had very similar enthalpy of adsorption, while the weaker affinity of PC was mainly from its higher entropy decrease. It is likely that PC is a larger molecule and has a higher entropy. After adsorbing on the surface, the flexibility of the choline part was also affected causing more entropy loss. Individual dimethyl phosphate is too weak to measure, and we did not try to fit the data. It is interesting to note that the phosphate groups in DNA were similar to that in DMP with two caps. Therefore, polyvalent binding must be important for the phosphate in DNA to bind. Studying the adsorption of phosphate species by metal oxides has been extensively carried out, but previous work focused on simple Pi, PPi or their derivatives.40,

51

Various spectroscopic methods and DFT calculations have been

performed to achieve atomic level understanding. In our work here, we focused on the thermodynamic and kinetic aspects of the reactions as a function of phosphate chain 19


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length and capping groups. Such understanding is complementary to the prior knowledge on this system.

Figure 6. (A) Kinetics of DNA displacement from CeO2 by phosphate with capping groups (10 µM) in HEPES buffer. ITC traces and integrated heat with fitting of one-site binding model for (B) PO43-; (C) PC; and (D) DMP (1.0 mM into 0.10 mg/mL CeO2).

Conclusions In summary, we have systematically studied the adsorption of various phosphate species on CeO2 nanoparticles using both a DNA-based fluorescent displacement assay and ITC. In general, the longer the phosphate chain, the higher adsorption affinity and the faster the kinetics of displacing DNA. The exception was STMP, which has a circular structure. Although STMP can tightly adsorb on CeO2, it cannot efficiently displace the adsorbed DNA likely due to kinetic issues related to its ring structure. At the same molar concentration of total phosphate unit, the highest desorption was observed with PPi and STTP, and this can be explained by the higher 20


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displacing power of terminal phosphate and its balance with the polyvalent effect. ITC provided a more quantitative binding picture and the similar total phosphate adsorption capacity for most species has indicated a tight lengthwise packing of these molecules. The difference in the STMP and STTP for DNA displacement was used to monitor the hydrolysis of STMP. By capping the phosphate with organic groups, the adsorption affinity was only slightly affected with one cap but significantly affected by two caps. Therefore DNA and lipids can tightly adsorb on CeO2 and other oxides relying on polyvalency.

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Acknowledgement We thank Dr. S. Jain for proofreading the paper. Funding for this work was from The Natural Sciences and Engineering Research Council of Canada (NSERC). Dr. X. Wang was supported by the Shandong Provincial Government Scholarship (China) to visit University of Waterloo. We also thank the Natural Science Foundation of Shandong Province, China (No. ZR2018MB030).

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For Table of Content Graphics Only

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Adsorption of Phosphate and Polyphosphate on Nanoceria Probed by

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