adsorption of drugs on nanodiamond: toward development of a drug delivery platform

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Article

Adsorption of Drugs on Nanodiamond: TowardsDevelopment of a Drug Delivery Platform

Vadym N. Mochalin, Amanda Pentecost, Xue-Mei Li, Ioannis Neitzel,Matthew Nelson, Chongyang Wei, Tao He, Fang Guo, and Yury Gogotsi

Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/mp400213z • Publication Date (Web): 14 Aug 2013

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Adsorption of Drugs on Nanodiamond: Towards Development of a Drug

Delivery Platform

Vadym N. Mochalin1, Amanda Pentecost

1, Xue-Mei Li

2, Ioannis Neitzel

1, Matthew Nelson

1,

Chongyang Wei2, Tao He

2, Fang Guo

2, Yury Gogotsi

1*

1 – A. J. Drexel Nanotechnology Institute and Department of Materials Science and Engineering,

Drexel University, Philadelphia, PA 19104, United States

2 – Shanghai Advanced Research Institute, Chinese Academy of Science, 99 Haike Road, Zhang-

Jiang Hi-Tech Park, Pudong, Shanghai 201210, China

* - corresponding author [email protected]

KEYWORDS: nanodiamond, adsorption, drug delivery, doxorubicin, polymyxin B, antibiotics

Abstract

Nanodiamond particles produced by detonation synthesis and having ~5 nm diameter possess

unique properties, including low cell toxicity, biocompatibility, stable structure and highly

tailorable surface chemistry, which render them an attractive material for developing drug

delivery systems. Although the potential for nanodiamonds in delivery and sustained release of

anticancer drugs has been recently demonstrated, very little is known about the details of

adsorption/desorption equilibria of these and other drugs on/from nanodiamonds with different

purity, surface chemistry, and agglomeration state. Since adsorption is the basic mechanism most

commonly used for the loading of drugs onto nanodiamond, the fundamental studies into the

details of adsorption and desorption on nanodiamond are critically important for the rational

design of the nanodiamond drug delivery systems capable of targeted delivery and triggered

release, while minimizing potential leaks of dangerous drugs. In this paper we report on a

physical-chemical study of the adsorption of doxorubicin and polymyxin B on nanodiamonds,

analyzing the role of purification and surface chemistry of the adsorbent.

Introduction

Diamond nanoparticles (nanodiamond, ND), commercially produced by detonation, offer a

unique combination of outstanding optical properties, biocompatibility, low toxicity, a

chemically inert core, and tunable robust surface structure and chemistry.1 Due to these unique

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properties, the interest in ND has been growing steadily during the past decade. Additionally,

NDs contain NV centers, which render them fluorescent2. A bright fluorescence can also be

produced by ND surface modification.3-4 Recent advances resulted in an intense development of

biomedical applications for NDs, including biomedical imaging, tissue engineering, and drug

delivery systems.5-16 Currently, ND is being mainly investigated for the delivery and sustained

release of anticancer chemotherapeutics,7 nucleic acids,8 and insulin.11 Due to its rich surface

chemistry,1, 17 different mechanisms, including adsorption and covalent binding18-19 of drugs can

be employed with ND depending on which better suits the purpose of a particular drug delivery

system being developed.

Adsorption/desorption mechanism is, in general, a first choice for drug delivery due to its

relative simplicity, universality, minimal changes to a drug structure (no alteration to chemical

structure of the drug molecule is needed, in contrast to covalent binding), lesser concerns about

changed bioactivity of the drug, and potential to easily create smart environment- and stimuli-

responsive ND drug delivery and release systems.20-22 Additionally, the adsorption of poorly

soluble drugs on the surface of well-dispersed biocompatible ND particles appears very attractive

for overcoming their poor bioavailability, which is a common issue for many drugs. In particular,

when adsorbed as a monolayer onto a 5 nm ND particle, the drug is maximally exposed to an

aqueous environment, even more than when it is administered in the nanocrystalline form.23

However, the monolayer capacity of ND for a particular drug must be determined in order to

gain full advantage of the mechanism while avoiding excessive loading and undesirable potential

leaks of the adsorbed drug. These potential leaks of adsorbed drugs will be especially dangerous

in case of anticancer chemotherapeutics, a vast majority of which are highly toxic for both

normal and cancerous cells. In this situation, in addition to the monolayer capacity, knowledge of

another parameter of adsorption, the binding strength of the drug to ND, becomes increasingly

important. Although anticancer chemotherapeutic delivery with ND is currently a hot topic of

research,5, 7-10, 14, 16 very little is known about ND monolayer capacity for these chemotherapeutic

drugs, their binding strength to ND, heat and kinetics of adsorption, the role of pH, ionic

composition, as well as ND purity, dispersion state, surface chemistry, and other important

parameters, critical for developing any practical ND system for delivery of anticancer or other

types of the drugs, for example, antibiotics. Due to its importance for triggered drug release, pH-

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dependent adsorption of doxorubicin on ND has been studied in a handful of papers21-22 but no

detailed experimental studies on adsorption of other types of drugs on NDs have been published.

Since the early years of ND research, it has become clear that this material should have

outstanding adsorption properties and indeed, ND has since been investigated for blood

cleansing,24 sorption of heavy metals,25 dyes,26-27 proteins,28-30 toxins,31-33 and even viruses.34 It

should be noted, however, that nearly all of these investigations have been application driven and

thus pursued the corresponding goals (i.e., aimed at concept demonstration or measuring the

maximal effect) while very little has been done towards systematic fundamental studies of

adsorption phenomena involving ND. Furthermore, NDs used in these studies were often not

well purified, de-agglomerated, and characterized, which makes the understanding of the

contributions of these factors in the adsorption process or optimization of adsorption/desorption

based on the published results nearly impossible.

Towards this end, a detailed physical-chemical studies of adsorption/desorption

thermodynamics and kinetics of different drugs on ND with different purity, agglomeration state,

and surface chemistry, while varying environmental parameters such as temperature, pH, ionic

composition of the solution, etc., are required. The goal of this study is not to design a particular

drug delivery system, but rather to focus on one of the most important steps on the road to

building any such system – on the adsorption of drugs onto ND. Here we report initial results on

the adsorption of doxorubicin (DOX), an anticancer drug and polymyxin B (PMB), an antibiotic

widely used to kill resistant Gram-negative infections, on NDs with different surface chemistries.

Experimental Part

Materials and characterization. Nanodiamond powders from two suppliers were used in this

study: UD90 (NanoBlox, Inc. U.S.A.) and ZH (Nanodiamond grey powder, Zhongchuan

Heyuan, China). In addition to as-received NDs, we studied the adsorption on purified and

surface modified NDs. Purification was done by oxidation of NDs in air at 425-430 ºC for 2 h.35

The purified NDs have 95-97 % wt. of sp3 carbon (diamond phase) and are terminated by oxygen

containing (mainly COOH) groups, hence we labeled them as UD90-COOH and ZH-COOH to

emphasize their surface chemistry. To study the effects of surface chemistry, we also produced

aminated NDs (UD90-NH2 and ZH-NH2) by covalent binding of ethylenediamine to COOH

terminated NDs via amide bonds (see ref.36 for details). Those chemical modifications produced

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the same surface chemistry on powders from different suppliers making them almost identical

from the standpoint of adsorption. Adsorption of DOX has been studied on ZH, ZH-COOH, and

ZH-NH2. Adsorption of PMB has been studied on UD90 and UD90-NH2. Raman measurements

were performed using an inVia+Reflex spectrometer (Renishaw, U.K.) with 514.5 nm excitation

wavelength (Ar-ion) laser. Fourier transform infrared (FTIR) spectra were recorded in the range

400 – 4000 cm-1 with 0.5 – 4 cm-1 resolution using an FTIR Prestige 21 spectrophotometer

(Shimadzu). The spectra were recorded in KBr pellets, prepared by pressing mixtures of ~200

mg KBr and ~1.5 mg ND under a load of 9 tons. XRD analysis (CuKα radiation, λ =1.54056 Å)

was performed using a D8 Advance X-Ray diffractometer (Bruker).

Adsorption measurements. DOX: ~10 mg of ZH, ZH-COOH, or ZH-NH2 was added to 5 mL

aqueous solutions of varied known concentrations of doxorubicin hydrochloride (reagent grade,

> 98 % pure, Sangon Biotech (Shanghai) Co, Ltd.) in 10 mL glass vials. The vials were closed

with plastic threaded caps, sonicated in an ultrasonic bath for 2 min, and shaken at ambient

temperature (25 ºC) overnight to ensure equilibrium adsorption. The resulting mixture was

centrifuged at 13300 rpm for 1 hr to ensure that all of the ND had been removed before

absorbance measurements were made. The peak absorbance of non-adsorbed DOX in solution

was measured at 480 nm for each trial with a 2802 UV-Visible scanning spectrophotometer

(Unico) and related to the DOX concentration by Beer’s Law using a linear calibration produced

in a series of separate measurements on the solutions with known concentrations of the drug and

no ND added (see Supporting Information). PMB: Polymyxin B sulfate (Fisher BioReagents)

was purchased from Fisher Scientific, U.S.A. ~20 mg of UD90 or UD90-NH2 was added to 50

mL aqueous solutions of varied known concentrations of PMB, sonicated in an ultrasonic bath

for 2 min, and magnetically stirred overnight at 25 ºC with a Teflon coated stirrer bar to ensure

equilibrium adsorption. The resulting mixtures were centrifuged at 3500 rpm for 2 – 6 hr, then

passed through a 20 nm pore diameter alumina syringe filter (Anapore, Whatman) to ensure that

all of the ND particles had been removed from solutions before absorbance measurements were

made. The peak absorbance of the supernatant was measured at 205 nm for each trial with an

Evolution 600 UV-Visible spectrophotometer (Thermo Scientific) and then related to the PMB

concentration through a calibration curve (see Supporting Information). The experimental

adsorption data for both drugs (the complete dataset is available in Supporting Information) were

fitted to the isotherm equations using a non-linear least squares procedure.

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Results and Discussion

Details of ND purification procedure and characterization of UD90 and UD90-COOH have

been reported elsewhere,35 as well as the amination of UD90 and characterization of

UD90-NH2.36 X-Ray Diffraction (XRD) patterns, as well as Raman and FTIR spectra of ZH,

ZH-COOH, and ZH-NH2 are shown in Figure 1.

Figure 1. XRD pattern of ZH (a); Raman spectrum of ZH (b); and FTIR spectra of ZH, ZH-

COOH, and ZH-NH2 (c, d).

The XRD pattern of ZH is typical for a good quality ND showing (111) and (220) peaks of

diamond lattice as well as a trace level (002) peak of graphite. The average crystal size of ZH

particles determined from the full width at half maximum of the diamond (111) peak D(111) in

Figure 1a by Scherrer equation is 3.9 nm, a little smaller than ~4.7 nm measured for UD90.37

Visible (λexc. = 514.5 nm) Raman confirms the high content of diamond (peak at 1329 cm-1) in

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ZH. Since the Raman cross-section of sp3 carbon for visible light is much smaller than that of sp2

carbon, the diamond peak of ND in visible Raman spectrum is usually very small35, 38,

necessitating the need to use UV Raman analysis.39 The diamond peak in the visible Raman

spectrum of ZH is more pronounced than usually observed in these conditions for other NDs,

including UD90.35 The peak at 1619 cm-1 in Figure 1b is a combined peak of sp2 carbon (the G-

band) and OH groups chemically bonded and (or) adsorbed on the ZH surface.39 The shoulder at

1712 cm-1 is a contribution of C=O containing surface functional groups of ZH.39 Changes in

surface chemistry of ZH after its oxidation (ZH-COOH) and amination (ZH-NH2) are illustrated

by FTIR spectra in Figure 1c, d and are similar to those observed for UD90, UD90-COOH, and

UD90-NH2 (all reported before, see refs.35-36, 39). As-received ZH shows more pronounced peaks

of OH stretching and bending vibrations and a suppressed C=O stretch peak as compared to

UD90.35 It also shows C-H stretch vibrations in CH2 and CH3 groups. In contrast to UD90, the

surface of ZH may be predominantly terminated by OH groups. As expected, air oxidation

removes CH2 and CH3 groups and creates a significant amount of C=O containing (mainly

COOH) functional groups on the ND surface, which give rise to a much stronger C=O stretch

band in ZH-COOH (Figure 1c). Amination of ZH-COOH via attachment of ethylenediamine

through amide bond36 results in corresponding changes in the spectrum of ZH-NH2. In particular,

the intensity of the C=O stretch of COOH is reduced, and new bands appear corresponding to

amines and secondary amides (Figure 1d). In the spectrum of ZH-NH2, an amide I (sec.) band

overlaps with and appears as a shoulder of the OH bending peak in the range 1640 – 1680 cm-1,40

and a smaller amide II (sec.) peak appears at 1530 – 1570 cm-1.40 These peaks are absent in both

ZH and ZH-COOH (Figure 1d) and provide a clear evidence of an amide bond formation

between one of the aminogroups of ethylenediamine and a COOH group of ND as described in

ref.36 Typical for amides and amines, there is also a peak of the C-N stretch at 1020 – 1220 cm-1.

This peak, although of smaller intensity, is also present in ZH and ZH-COOH, as well as in other

detonation NDs, where it is usually associated with NV defects in the ND structure.1

Measurements of the drug concentrations in solution before the addition of ND (��) and in

equilibrium with ND (��) reveal pronounced differences in the way the drug is adsorbed by

NDs. The plots of the percent of drug removed from solution, �� ⁄ ∙ 100, vs. ��

(Figure 2) show that 95-98 % of DOX is adsorbed at low concentrations by ZH, ZH-COOH or

ZH-NH2. However, at higher DOX concentrations these NDs behave very differently. While ZH-

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NH2 adsorbs more than 90 % of DOX from its solutions at concentrations 0.1 – 0.5 mg/mL, ZH-

COOH adsorbs more than 95 % of DOX only from its diluted solutions (up to 0.15 mg/mL)

leaving significantly more drug in solution at higher DOX concentrations. ZH adsorbs 96 % of

DOX from its most diluted solution (0.007 mg/mL), whereas, even at slightly higher DOX

concentrations, the percent removal of the drug by ZH drops significantly to 13 % at 0.16 mg/mL

of DOX. The corresponding measurements for PMB and UD90 and UD90-NH2 reveal very

different results. Both UD90 and UD90-NH2 can remove no more than 70-75 % of PMB from

aqueous solutions, and this is at the lowest concentrations of PMB, below 0.04 mg/mL for UD90

and below 0.02 mg/mL for UD90-NH2. Any further increase in �� of PMB results in reduction of

its removal down to 24-30 % at �� of PMB > 0.09 mg/mL for UD90-NH2 and > 0.12 mg/mL for

UD90, respectively.

Figure 2. Percent removal of DOX (a) and PMB (b) from aqueous solutions of initial

concentrations � in equilibrium with different NDs.

The adsorption isotherms measured in this study are presented in Figure 3. Although no

published results on adsorption of these drugs by ND are available, the experimental values of

adsorption (Figure 3a-e) agree reasonably well with data on other adsorbates, which can be

found in literature (e.g. refs.26, 29, 41-42, etc.). The experimental data were fit by non-linear least

squares method to two commonly used adsorption isotherms, Langmuir and Freundlich (Table 1,

Figure 3a-e), with the best fit parameters summarized in Table 1.

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The Langmuir isotherm (eq 1),43 originally developed to describe gas-solid adsorption, has

nevertheless been traditionally used to quantify and contrast the performance of different

sorbents in liquid-solid adsorption processes. This empirical model assumes monolayer

adsorption by a finite number of localized identical adsorption sites on the surface of the

adsorbent. Any possible interactions (including steric hindrance) between the adsorbed

molecules, even between the molecules occupying adjacent adsorption sites, are excluded.

� � �� ∙�� ∙ ��

1 � �� ∙ �� (1),

where � is adsorption capacity (mass of adsorbate per mass of adsorbent), �� is maximal

adsorption capacity of monolayer (mass of adsorbate per mass of adsorbent), �� is a constant

which characterizes the strength of binding of the adsorbate to the adsorbent, �� is concentration

of the adsorbate in the environment in equilibrium with the adsorbent.

Figure 3. Adsorption isotherms of DOX and PMB on NDs with different surface chemistries, (a-

e) (fit by Langmuir (solid blue lines) and Freundlich (dashed red lines) models); and ζ-potential

of UD90 and UD90-NH2 in PMB solutions depending on the equilibrium concentration of PMB

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The Langmuir isotherm refers to homogeneous adsorption, i.e. all molecules of the adsorbate

have identical and constant enthalpy of adsorption and activation energy of adsorption or in other

words, all adsorption sites possess equal affinity for the adsorbate.

Table 1. Best fit parameters of Langmuir and Freundlich isotherms for adsorption of DOX

and PMB on NDs with different surface chemistries

ND type Langmuir isotherm (eq 1) Freundlich isotherm (eq 2)

��, mg/g ��, mL/mg R a ��

b � b

R a

Doxorubicin

ZH 10.49 175.7 0.84 16.76 4.82 0.89

ZH-COOH 87.36 257.3 0.98 161.8 3.97 0.93

ZH-NH2 288.4 52.63 0.96 445.5 3.02 0.91

Polymyxin B

UD90 92.30 172.0 0.95 145.7 5.02 0.93

UD90-NH2 53.02 331.2 0.96 79.66 6.49 0.91 a – Pearson’s correlation coefficient,

b – values calculated for C expressed in mg/mL and A in mg/g

The Freundlich isotherm44 is one of the earliest quantitative relationships between the

adsorption and equilibrium concentration of the adsorbate (eq 2).

� � �� ∙ �� ! "⁄ # (2),

where � is adsorption capacity (mass of adsorbate per mass of adsorbent), �� and $ are empirical

constants, �� is concentration of the adsorbate in the environment in equilibrium with the

adsorbent.

This empirical relationship is not restricted to the formation of monolayer and is used to

describe adsorption on heterogeneous surfaces with non-uniform distribution of adsorption

energy and affinity over a population of adsorption sites. Although criticized for its lack of a

fundamental thermodynamic basis, this model is often used for fitting complex solid-liquid

adsorption equilibria, especially when the adsorbents have heterogeneous surfaces or the

adsorbate is a mix of many components (vegetable oil, natural pigments, etc.). In contrast to the

(f) (the inset shows ζ-potential of these NDs at very low concentrations of PMB).

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Langmuir model, the Freundlich isotherm has no saturation limit (no monolayer) since, at any

equilibrium concentration within the range of its applicability, the value of adsorption is a sum

over all adsorption sites, each with different affinity, where the sites of stronger affinity are

occupied first followed by the sites of weaker affinity, corresponding to the exponential decay in

the energy of adsorption.

As seen in Figure 3 and Table 1, the adsorption data demonstrate an overall better agreement

with the Langmuir model in the range of concentrations studied. ND particles are nearly

spherical in shape, i.e., geometrically all adsorption sites on their surface are nearly equivalent,

especially when compared to other shapes, e.g. cylinders (carbon nanotubes), flakes (clays and

other 2-D materials), or irregular bodies with intricate systems of pores (activated carbons,

zeolites). Therefore, one expects NDs to demonstrate less geometrical surface heterogeneity

compared to traditional porous adsorbents. This reasoning may explain a better fit of adsorption

data for NDs by the Langmuir model. Still, adsorption sites on NDs can be heterogeneous

chemically, due to the variety of numerous functional groups exposed on their surfaces. In

addition, as-received NDs contain considerable amounts of graphitic and amorphous carbon that

may form micropores and bring about higher geometrical heterogeneity. Table 1 shows that both

as-received NDs (UD90 and ZH) have slightly lower correlation coefficients of Langmuir fit, as

opposed to purified and chemically modified powders. Purification reduces both geometry and

chemistry related sources of adsorption heterogeneity by removing microporous non-diamond

carbon (yielding particles of more uniform shape) and converting surface functionalities into

oxygen containing, mainly COOH groups (resulting in more homogeneous surface chemistry).

Subsequent amination uniformly changes the ND surface from COOH terminated into NH2

terminated without adding chemical heterogeneity. Thus, the Langmuir model better fits the data

for purified and surface modified NDs because they are cleaner and more homogeneous in terms

of their surface chemistry. Interestingly, in recent studies,26, 42 where as-received NDs from a

different supplier were used without further purification, the data were fitted by the Freundlich

isotherm, suggesting heterogeneous adsorption. On the other hand, in those studies where

purified and/or modified NDs were used for adsorption experiments (e.g., refs.27, 29, 41), the

results could be fitted with the Langmuir equation at least for smaller adsorbates at low

concentrations, where the effects of adsorbate association in solution are not significant. Of

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course, in addition to shape, purity, and chemistry, the effects of ND agglomeration in

suspensions need to be taken into consideration in future, more detailed, analyses of adsorption.

The data in Table 1 clearly demonstrate the profound effect of ND surface chemistry on its

adsorption characteristics. Depending on the surface terminations of NDs, the monolayer

capacity (�� in eq 1), varies between 10 and 288 mg of DOX and between 53 and 92 mg of

PMB per 1 g of ND (Table 1). The strength of binding (�� in eq 1) also varies in a wide range

depending on the ND surface termination. The separation factor, or equilibrium parameter, %�,

can be calculated from the Langmuir isotherm as (eq 3)

%� �1

1 � �� ∙ �� (3),

where ��is the adsorbate initial concentration.

This parameter is related to the shape of the isotherm and can be used to define the

adsorption process as irreversible when %� � 0, favorable when 0 & %� & 1, linear when

%� � 1, and unfavorable when %� ' 1.45 The plots of %� as a function of initial concentration of

the adsorbate are presented in Figure 4. At any concentration for both DOX and PMB, the

calculated %� values are between 0 and 0.5, indicative of favorable reversible adsorption, which

is beneficial for a ND drug delivery system.

Figure 4. Equilibrium parameter R) as a function of initial concentration of adsorbate in

solution, C0, for DOX (a) and PMB (b) adsorbed on different NDs.

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Analysis of the ζ-potential of UD90 and UD90-NH2 in equilibrium with PMB solutions of

different concentrations (Figure 3f) suggests that the electrostatic interactions between the

cationic PMB molecules and ND surface are largely responsible for the more than two times

higher monolayer capacity of UD90 as compared to UD90-NH2. The ζ-potential of UD90-NH2

in aqueous dispersions is 2.2 mV more positive compared to UD90 due to protonation of NH2

groups in the former material. When PMB is added to the system in very low concentrations

(between 0 and 0.02 mg/mL), the ζ-potential of UD90 and UD90-NH2 jumps from negative to

high positive values by more than 20 mV and, afterwards, remains essentially constant at higher

PMB concentrations up to 0.1 mg/mL. Comparison of Figure 3f to Figure 3d,e shows that the

sharp change in the ζ-potential for the both types of ND corresponds to the range of PMB

concentrations where a monolayer of the adsorbate is formed. Since UD90-NH2 dispersed in DI

water (pH ≤ 7) is protonated and exposes positively charged NH3+ groups on its surface (see

ref.36, also evidenced by its more positive ζ-potential compared to UD90 in Figure 3f) and PMB

molecules have a net positive charge at this pH, we ascribe a significantly reduced PMB

monolayer capacity of UD90-NH2 (Figure 3e), as compared to UD90 (Figure 3d), to electrostatic

repulsion between the NH3+ groups of UD90-NH2 and PMB molecules.

Comparison of �� and �� (eq 1) for different adsorbates and NDs (Figure 5) is quite

Figure 5. �� vs. �� from Langmuir model for doxorubicin and polymyxin B adsorbed on NDs

with different surface chemistry (data from Table 1).

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instructive in revealing further details about adsorption and providing practical insights for

developing ND enabled drug delivery systems. A remarkably high DOX monolayer capacity is

achieved with ZH-NH2, and, interestingly, this same ND has the lowest binding strength towards

DOX (Figure 5). Due to protonation of NH2 group present in DOX molecule, it is assumed to

have one unit of positive charge in DI water. Thus, it should be repelled by the similarly charged

ZH-NH2 surface. Nevertheless, ZH-NH2 shows a very high monolayer capacity for DOX. This

high value of �� and low �� for DOX adsorbed onto ZH-NH2 may indicate that other

interactions (hydrogen bonding between the OH groups of DOX and NH2 groups of ZH-NH2, as

well as hydrophobic ND-DOX interactions), weaker than electrostatic interactions may play

more important role in this case (hence low ��). Furthermore, the maximal binding strength of

PMB achieved with UD90-NH2 corresponds to a moderate monolayer capacity. A potential

reason for this could be a change in the peptide conformation caused by strong electrostatic

interactions between the UD90-NH2 and PMB and resulting in a change in the area of the

molecule facing ND surface. The origin of this mismatch in the parameters of adsorption is not

clear and needs further study. However, regardless of origin, this mismatch of �� and ��,

which has also been observed before for other NDs and adsorbates,41 may prove to be very

important for the development of ND drug delivery systems. Low binding strength (�� in eq 1),

meaning easier drug release, will be beneficial in those cases when long-term storage of tightly

bonded drug in the body is not required before it is released, and when potential leaks of the drug

do not constitute a serious problem (e.g., for relatively non-toxic and poorly soluble drugs). On

the contrary, when drugs are highly toxic (e.g., DOX) and should be released from ND only after

delivery to the target sites, while minimizing drug leaks before the ND-drug complex has

reached the location, one may have to sacrifice the adsorption capacity in favor of a higher

binding strength. In this scenario, ZH-COOH will be the preferred choice over ZH-NH2 for

DOX, based on Figure 5. This analysis provides just one example of how the fundamental

understanding of adsorption mechanisms can be translated into practical recommendations.

Figure 5 and Figure 2 also show that the differences in the removal of the drugs from aqueous

solutions by NDs with different surface chemistries are due to the different values of ��.

Besides drug delivery, sorption properties of ND are critically important for chromatographic

separation and extraction,46-48 mass-spectrometry,49 and other applications.50-52 Therefore, similar

studies should be conducted on a variety of practically relevant adsorbates and NDs.

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Conclusions

Studies of equilibria between aqueous solutions of doxorubicin or polymyxin B with NDs in

isothermal conditions, reported herein, reveal profound differences in the adsorption of these

drugs on ND with different purity and surface chemistry. Although the adsorption of both drugs

on all studied NDs follows Langmuir isotherm and can be characterized as favorable reversible

adsorption based on the separation factors %� and therefore being beneficial for drug delivery,

the parameters of adsorption such as the maximal monolayer capacity, ��, binding strength,

��, as well as the degree of drug removal from solution demonstrate strong dependence on ND

surface chemistry. In some cases, the differences can be explained based on the atomic structures

of the drugs and NDs; for example, the twofold decrease in �� for polymyxin B adsorbed on

ND after amination as compared to the as-received ND, can be related to electrostatic repulsion

between similarly positively charged molecules of the adsorbate and the ND-NH2 particles. In

other cases, the observed differences invite further studies in order to be fully explained. In

particular, the highest �� for doxorubicin is achieved with the aminated nanodiamond, which,

however, has the lowest �� for doxorubicin; in contrast, the �� of aminated nanodiamond for

polymyxin B is lower compared to the as-received ND, while the �� is higher. Although we

currently do not have a satisfactory explanation of these mismatches in the �� and ��, it is

important to realize that they exist and can be used to the benefits of various ND drug delivery

systems. Our work not only provides the adsorption characteristics of particular drugs on

particular NDs, but also emphasizes the importance of future detailed studies of the mechanisms,

thermodynamics, and kinetics of adsorption of different drugs on ND with different purity,

surface chemistry and agglomeration state at different temperatures, pH, ionic strength, and other

parameters of environment, for rational design of ND enabled drug delivery platforms.

Supporting Information Available

Raw adsorption data and calibration curves for the drugs in aqueous solutions. This

information is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements

This work was supported by the Drexel-SARI Center. We acknowledge experimental help

from Dr. Guanming Chen (SARI). We thank the Nanomedicine and Translational Medicine and

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15

the Sustainable Technology Centers at SARI for use of their lab equipment as well as

instruments for UV-Visible, FT-IR, and XRD analyses. We also appreciate the use of the Raman

spectrometer at the East China University of Science and Technology.

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Table of Contents Graphics

0 20 40 60 800

20

40

60

80

100

Ad

so

rptio

n (

mg P

oly

myxin

B /

g N

D)

Equilibrium Conc. of Polymyxin B (mg/L)

ONH

NH3+

ONH OHO

NHNH3

+

ONHNHO

NHO

NH3

+NH

O

NH3

+

NHO

NHO

NHO

NH3

+

NH

O

OH

O

O-

O

O-

O

O-

O

O-

O

O-

Polymyxin B

O

O-

O

O-

O

O-

O

O-

O

O-

NH3+

NH3+

ONH

NH3+

ONH OHO

NHNH3

+

ONHNHO

NHO

NH3

+NH

O

NH3

+

NHO

NHO

NHO

NH3

+

NH

O

OH

ND

-NH

2

ND

-CO

OH

NH3+

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adsorption of drugs on nanodiamond: toward development of a drug delivery platform

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