# Superparamagnetic Ironoxide Nanoparticles via Ligand Exchange Reactions: Organic 1,2-Diols as  Versatile Building Blocks for Surface Engineering

Authors: Wolfgang H. Binder; Harald Weinstabl; Robert Sachsenhofer
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

A method for the preparation of ligand-covered superparamagnetic iron oxide nanoparticles via exchange reactions is described. 1,2-diol-ligands are used to provide a stable binding of the terminally modified organic ligands onto the surface ofγ-Fe2O3-nanoparticles (r∼4 nm). The 1,2-diol-ligands are equipped with variable terminal functional groups (i.e., hydrogen bonding moieties, azido- bromo-, fluorescent moieties) and can be easily prepared via osmium tetroxide-catalyzed 1,2-dihydroxylation reactions of the corresponding terminal alkenes. Starting from octylamine-covered γ-Fe2O3-nanoparticles, ligand exchange was effected at 50∘C over 24–48 hours, whereupon complete ligand exchange is taking place as proven by thermogravimetric (TGA)- and IR-spectroscopic measurements. A detailed kinetic analysis of the ligand exchange reaction was performed via TGA analysis, demonstrating a complete ligand exchange after 24 hours. The method offers a simple approach for the generation of various γ-Fe2O3-nanoparticles with functional organic shells in a one-step procedure.


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## Body

## 1. Introduction

Magnetic iron oxide
nanoparticles have  gained  increased  interest during the past decade, mostly due
to their superparamagnetic property present at small particle sizes [1, 2]. Thus
below a critical size of ~40 nm, the formation of Weiss
domains is no longer possible, leading to  the formation of singular
magnetic domains and thus the physicochemical phenomenon of superparamagnetism.
With the advent of this discovery, numerous exploitations have gained advantage
of this effect, mostly due to the inability of superparamagnetic nanoparticles
to self-aggregate, thus preventing large agglomerate formation. Applications such
as drug and gene
delivery [3, 4], magnetic resonance imaging [5], hyperthermia [6–8], and magnetofection
[9–11] have demonstrated the wide
applicability of superparamagnetic nanoparticles in vivo and in vitro.A critical issue for the use of
these nanoparticles is represented by engineering the surface of the
nanoparticles with organic ligands, thus allowing the incorporation, selective
recognition, or specific guidance of such nanoparticles. Based on various
approaches, a large number of different organic ligands have been described,
including ligands such as polymeric [12–14],
peptidic [15, 16], protein ligands [17–19], or small
organic molecules such as fluorescent dyes [20, 21], biotine
[22], drugs, and others [23, 24]. Critical for the
achievement of a sufficient stability between the surface of the nanoparticles
and the corresponding ligands is the choice of either a multivalent adsorption
site (usually present between polymeric ligands and the surface of the
iron-oxide nanoparticles) or the use of a specific functional group, binding tightly to the surface of the iron
oxide nanoparticles. Inorganic shells, such as silicon
dioxide [25–29] have
been used extensively as an intermediate shell between the iron-oxide core and
the organic ligands, usually binding via terminal aminoligands incorporated on
the surface of the silica shell. When purely organic ligands are directly bound
to the surface  of the iron oxide nanoparticles, carboxylate ligands [30, 31], sulfonates and thiols [32, 33],
phosphates and phosphonates [34], as well as aminoligands [35] are used. Recently, Boal et al. [36] have introduced the use
of 1,3-diol-ligands for achieving a tight binding of various organic ligands on
the iron oxide nanoparticle surface. 1,3-diols were found superior as ligands
in comparison to pure alcoholic substrates, since binding constants of the
latter are not sufficient in order to achieve sufficient binding stability.In the present publication, we
report on the use of1,2-diol-ligands
instead of 1,3-diol-ligands for the
binding of various ligands to the surface of superparamagnetic iron oxide
nanoparticles (see Figure 1) via ligand exchange reactions. In contrast to
1,3-diols, 1,2-diols can be generated easily from the corresponding terminal
alkenes via an osmium tetraoxide mediated dihydroxylation reaction, thus
achieving a broad spectrum of substrates at the end of the ligand. The binding
of various ligands with fluorescent and supramolecular properties onto the
surface of the iron oxide nanoparticles together with the kinetic progress of the
ligand exchange reaction is
reported, allowing a simple and efficient functionalization of the iron oxide
nanoparticles.Figure 1
Modification of
octylamine-covered Fe2O3-nanoparticles NP1 (r = ~4 nm) via ligand exchange
reactions using terminally substituted 1,2-diol-ligands.


## 2. Experimental Section

### 2.1. General Procedures

Sodium azide (NaN3)
was from Acros and Cu(I)Br (99%) from Fluka. All chemicals were used  directly
without any further purification. Tetrahydrofuran (THF), diethylether, and
toluene were dried by distillation over sodium and benzophenone,
dimethylformamide (DMF) over CaH2, and methanol over magnesium. NMR
spectra were obtained with a 200 MHz Bruker AC200 spectrometer and a 400 MHz
Bruker Advance DRX 400 MHz. Chloroform (CDCl3) and dimethylsulfoxide
(DMSO-d6) were used as solvents, tetramethylsilane (TMS) as internal
standard. Photoluminescence spectra were recorded on a Perkin-Elmer luminescent
spectrometer LS 50 B system equipped with a pulsed Xe discharge lamp as the
excitation light source (400 nm) and a photomultiplier detector. UV-VIS
spectroscopy was performed on a Hitachi U-2001 spectrophotometer using UV
Solutions (Version 1.2, Build 122, Hitachi
instruments) for data interpretation. TEM measurements were achieved on a
Phillips EM300 microscope, using an accelerating voltage of 100 kV. A droplet
of the nanoparticle solution (0.1–1 mg/ml) was applied on standard TEM-grids
(300 mesh copper carbon only (50 ct)) purchased from Electron Microscopy
Sciences and dried at ambient conditions. DLS measurements were done on an
ALV/CGS-3 compact goniometer using the ALV-5000/E correlator software.


### 2.2. Synthesis

#### 2.2.1. Preparation of the 1,2-Diol-Ligands1a–1f by Osmium-Tetroxide Catalyzed Dihydroxylation Reaction of Terminal Olefines 2a–2f

The
corresponding olefin was suspended in the denoted amount of Ar-bubbled solvent.
NMO⋅H2O (N-methylmorpholine-N-oxide monohydrate) was added in the
specified amount. The solution was stirred at room temperature until all
reagents were dissolved. During agitation, the solution was bubbled with argon.
Now catalytic amounts (one granule) of OsO4 were added in Ar counterflow. The
solution turned immediately yellow after the addition. The reaction mixture was
stirred  at room temperature until TLC indicates full conversion, which in all
cases occurred after 1 hour. Since OsO4 is a very strong oxidizing
agent it has to be decomposed before workup. Therefore 50 mL of an aqueous 10 w% NaHSO3 solution were added. The reaction mixture turned violet and a precipitate
occurred, which was filtered. Phases were transferred into a separation funnel
and separated. The organic layer was washed twice with water and dried with
brine and Na2SO4. Solvents were removed under reduced
pressure.


#### 2.2.2. Data for the Ligands1a–1f

5-(10,11-Dihydroxyundecyl)-5-Ethylpyrimidine-2,4,6-Trione1a
The alkene2a (3 g, 9.73 mmol), NMO⋅H2O (3 g, 21.90 mmol), and OsO4 were
suspended in 50 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. The crude product was purified by
silica gel chromatography (EE/hexane = 3/1) yielding in 1a as a colorless oil (3 g, 90%). Rf = 0.48 (EE/hexane = 3/1). 1H-NMR (200 MHz, DMSO-d6) δ (ppm) = 11.57 (s, 2H), 4.40 (m, 2H), 3.26 (m,
2H), 1.83 (t, 4H) 1.39–1.23 (m, 16H), 0.77 (t, 3H). 13C-NMR (50 MHz, DMSO-d6) δ (ppm) = 174.12, 150.86, 72.06, 66.97, 56.71,
38.91, 34.38, 32.66, 30.26, 30.02, 29.84, 29.59, 26.16, 25.46.


#### 2.2.3. 11-Azidoundecane-1,2-Diol1b

Starting from the alkene2b (520 mg, 2.66 mmol), NMO⋅H2O (730 mg, 5.32 mmol) and OsO4 according to the general dihydroxylation procedure described above, 1b (586 mg, 96%) was obtained as
colorless oil after purification by silica gel chromatography (EE/hexane =
1/1). Rf = 0.30 (EE/hexane = 1/1).1H-NMR (200 MHz, CDCl3) δ  (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38 (m,
1H), 3.24 (t, 2H), 1.59 (m, 2H), 1.29 (m, 14H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 72.22, 66.60, 51.32, 32.93, 29.53, 29.32,
29.02, 28.69, 26.57,  25.50.


#### 2.2.4. 11-Bromoundecane-1,2-Diol1c

11-Bromoundec-1-ene2c (1 g, 4.29 mmol), NMO⋅H2O (1.3 g, 9.43 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated along the general
dihydroxylation procedure described above, yielding product 1c as a white solid (1.12 g, 98%). Rf = 0.44 (EE/hexane = 3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38
(m, 1H), 3.40 (t, 2H), 1.81 (m, 2H), 1.41 (m, 2H), 1.29 (m, 12H). 13C-NMR
(50 MHz, CDCl3) δ (ppm)
= 72.22, 66.60, 33.97, 33.02, 32.72, 29.30, 28.66, 28.05, 25.51.


#### 2.2.5. 5-(Dimethylamino)-N-(undec-10-Enyl)Naphthalene-1-Sulfonamide1d

Alkene2d (400 mg, 1 mmol), NMO⋅H2O (275 mg, 2 mmol), and OsO4 were reacted
in 20 mL dichloromethane and treated along the general dihydroxylation procedure
described above. The crude product was purified by silica gel chromatography
(EE/hexane = 3/1) yielding in 1d as
yellow green oil (410 mg, 94%). Rf = 0.23 (EE/hexane =
3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 8.54 (d, 1H), 8.27 (m, 2H), 7.55 (m, 2H),
7.19 (d, 1H), 4.71 (broad s, 1H), 3.63 (d, 2H), 3.42 (m, 1H), 2.89 (s, 6H),
2.04 (broad s, 2H), 1.38–1.10 (m, 18H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 134.92, 130.11, 129.70, 129.64, 129.55,
128.20, 123.56, 115.41, 72.26, 66.68, 45.56, 43.32, 33.74, 29.50, 29.26, 29.00,
28.91, 28.84, 26.37.


#### 2.2.6. 5-(10,11-Dihydroxyundecanamido)-N1,n3-Bis(6-Octanamidopyridin-2-Yl)isophthalamide 1e

The alkene2e (500 mg, 0.64 mmol), NMO⋅H2O (176 mg, 1.28 mmol), and OsO4 were dissolved in 15 mL dichloromethane and treated along the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1e was obtained
as white solid (512 mg, 98%). Rf = 0.31 (EE/hexane =
4/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 9.34 (s, 1H), 8.93–7.37 (m, 9H), 3.63 (d, 2H), 3.42 (m, 1H), 3.41
(t, 2H), 2.50–2.35 (m, 6H), 2.00 (m, 2H), 1.73 (m, 2H), 1.62
(m, 4H), 1.27 (m, 24H), 0.87 (t, 6H). 13C-NMR (50 MHz, CDCl3)
δ (ppm) = 173.03, 164.72, 156.23, 149.04,
141.07, 137.63, 134.57, 120.65, 120.53, 105.83, 75.44, 70.58 36.11, 33.86,
31.83, 29.69, 29.58, 28.94, 28.65, 28.63, 25.64, 22.73, 14.11.


#### 2.2.7. 4-(1-(Tert-Butyl(2-Methyl-1-Phenylpropyl)Aminooxy)Ethyl)Benzyl 10,11-Dihydroxyundecanoate1f

The alkene2f [37] (450 mg, 0.86 mmol), NMO⋅H2O (237 mg, 1.73 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1f was obtained
as colorless oil (461 mg, 96%). Rf = 0.46 (CHCl3/MeOH
= 20/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 7.42–7.15 (m, 18H), 4.12–3.16 (m, 10),
2.33 (t, J = 7.5 Hz, 4H), 1.63–1.32 (m, 15H), 2.00 (q, 4H), 1.55 (m, 11H), 1.25
(s, 16H), 1.05 (s, 9H), 0.96 (d, 3J = 6.2 Hz, 3H), 0.77 (s, 9H), 0.55
(d, 3J = 6.4 Hz, 3H), 0.22 (d, 3J = 6.4 Hz, 3H). 13C-NMR
(50 MHz, CDCl3) δ (ppm) = 169.08, 146.03, 145.31, 142.22,
142.11, 140.04, 136.44, 135.69, 130,82, 127.56, 126.73, 125.65, 121.37, 121.09,
119.13, 109.52, 108.77, 82.59, 81.68, 71.53, 66.48, 36.89, 35.08, 31.46, 31.17,
28.27, 27.86, 24.91, 24.22, 22.58, 22.08, 21.59, 20.55, 20.46.


#### 2.2.8. Preparation of Superparamagnetic Iron-Oxide NanoparticlesNP1

Preparation of
octylamine-covered nanoparticles was accomplished according to literature
[35, 36]. In brief, a solution of iron cupferron
complex (780 mg, 1.5 mmol) in octylamine (3.90 g, 27.22 mmol) was alternately evacuated to
10 mbar and aerated with Ar at 60°C. Trioctylamine (7.00 g, 18.33 mmol) was
treated the same way at 100°C. The degassed octylamine solution was rapidly
injected into the trioctylamine solution at 300°C. Immediately after the
injection, the reaction vessel was moved to a 225°C prewarmed bath and
remained there for 30 minutes under vigorous magnetic stirring. After the reaction
was stopped by removing the heating source, the particles were allowed to cool
to room temperature. They were dissolved in anhydrous toluene, precipitated by
the addition of dry methanol, centrifuged, and again dissolved in anhydrous
toluene. This washing step was repeated three times. The stabilized particles
were stored under Ar-atmosphere in anhydrous toluene. Nanoparticle sizes were
determined by DLS and TEM measurements. The stabilized NPs (NP1) stay soluble for several months,
indicating that no agglomeration takes place. IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2926 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1623 (−NH2), 1464 (−CH3, −CH2−).


#### 2.2.9. Ligand Exchange ofNP1 Leading to NP3a–NP3f

Surface
modification reactions were performed according to the general protocol for ligand exchange
reactions of Boal et al. [35, 36]. All
reaction steps were performed under Ar-atmosphere. 50 mg stabilized NPs (NP1) were dissolved in 30 mL of a 1:1
mixture of CHCl3 and toluene. A second mixture was prepared
consisting of 250 mg of diol-ligand 1a–f in a 1:1-mixture of chloroform and toluene. If the solubility of the ligand in
this mixture was too poor—which occurred
using ligands 1a, d, e—the smallest
amount of anhydrous isopropanol was added to dissolve the ligand. The ligand
solution was added to the nanoparticle solution via a syringe and the reaction
mixture was heated up to 50°C under vigorous stirring for 2 days. After ~48
hours, the exchange reaction was finished, resulting in surface modified
nanoparticles NP3a–f. The particles
were purified by repeated precipitation with dry methanol, centrifugation, and
resuspension in anhydrous toluene. Ligand exchange was monitored by FTIR measurements, analyzing the dried
modified nanoparticles as KBr-pellets.NP3a: IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2928 (−CH3, −CH2−), 2856 (−CH3,
−CH2−), 1702 (O=C–NH–C=O, six membered ring), 1450 (−CH3,
−CH2−), 1300 (−OH). NP3b:
IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 2094 (−N3), 1450 (−CH3, −CH2−). NP3c: IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2924 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1469 (−CH3, −CH2−), 775 (−CH2-Br). NP3d: IR (KBr-pellet) ν (cm-1)
3433 (−OH), 2956 (−CH3, −CH2−), 2916 (−CH3,
−CH2−), 2857 (−CH3, −CH2−), 1450 (−CH3,
−CH2−), 1373 (−SO2–NH−),
1150 (−SO2−). NP3e: IR
(KBr-pellet) ν (cm-1)
3420 (−OH), 2951 (−CH3, −CH2−), 2917 (−CH3,
−CH2−), 1628 (O=C–NR2), 1520 (O=C–NR2), 1450
(−CH3, −CH2−). NP3f:
IR (KBr-pellet) ν (cm-1)
3431 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 1740 (−COOR), 1456 (−N–O−), 1260 (−OH), 1030 (2 signals for
esters).


## 2.1. General Procedures

Sodium azide (NaN3)
was from Acros and Cu(I)Br (99%) from Fluka. All chemicals were used  directly
without any further purification. Tetrahydrofuran (THF), diethylether, and
toluene were dried by distillation over sodium and benzophenone,
dimethylformamide (DMF) over CaH2, and methanol over magnesium. NMR
spectra were obtained with a 200 MHz Bruker AC200 spectrometer and a 400 MHz
Bruker Advance DRX 400 MHz. Chloroform (CDCl3) and dimethylsulfoxide
(DMSO-d6) were used as solvents, tetramethylsilane (TMS) as internal
standard. Photoluminescence spectra were recorded on a Perkin-Elmer luminescent
spectrometer LS 50 B system equipped with a pulsed Xe discharge lamp as the
excitation light source (400 nm) and a photomultiplier detector. UV-VIS
spectroscopy was performed on a Hitachi U-2001 spectrophotometer using UV
Solutions (Version 1.2, Build 122, Hitachi
instruments) for data interpretation. TEM measurements were achieved on a
Phillips EM300 microscope, using an accelerating voltage of 100 kV. A droplet
of the nanoparticle solution (0.1–1 mg/ml) was applied on standard TEM-grids
(300 mesh copper carbon only (50 ct)) purchased from Electron Microscopy
Sciences and dried at ambient conditions. DLS measurements were done on an
ALV/CGS-3 compact goniometer using the ALV-5000/E correlator software.


## 2.2. Synthesis

### 2.2.1. Preparation of the 1,2-Diol-Ligands1a–1f by Osmium-Tetroxide Catalyzed Dihydroxylation Reaction of Terminal Olefines 2a–2f

The
corresponding olefin was suspended in the denoted amount of Ar-bubbled solvent.
NMO⋅H2O (N-methylmorpholine-N-oxide monohydrate) was added in the
specified amount. The solution was stirred at room temperature until all
reagents were dissolved. During agitation, the solution was bubbled with argon.
Now catalytic amounts (one granule) of OsO4 were added in Ar counterflow. The
solution turned immediately yellow after the addition. The reaction mixture was
stirred  at room temperature until TLC indicates full conversion, which in all
cases occurred after 1 hour. Since OsO4 is a very strong oxidizing
agent it has to be decomposed before workup. Therefore 50 mL of an aqueous 10 w% NaHSO3 solution were added. The reaction mixture turned violet and a precipitate
occurred, which was filtered. Phases were transferred into a separation funnel
and separated. The organic layer was washed twice with water and dried with
brine and Na2SO4. Solvents were removed under reduced
pressure.


### 2.2.2. Data for the Ligands1a–1f

5-(10,11-Dihydroxyundecyl)-5-Ethylpyrimidine-2,4,6-Trione1a
The alkene2a (3 g, 9.73 mmol), NMO⋅H2O (3 g, 21.90 mmol), and OsO4 were
suspended in 50 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. The crude product was purified by
silica gel chromatography (EE/hexane = 3/1) yielding in 1a as a colorless oil (3 g, 90%). Rf = 0.48 (EE/hexane = 3/1). 1H-NMR (200 MHz, DMSO-d6) δ (ppm) = 11.57 (s, 2H), 4.40 (m, 2H), 3.26 (m,
2H), 1.83 (t, 4H) 1.39–1.23 (m, 16H), 0.77 (t, 3H). 13C-NMR (50 MHz, DMSO-d6) δ (ppm) = 174.12, 150.86, 72.06, 66.97, 56.71,
38.91, 34.38, 32.66, 30.26, 30.02, 29.84, 29.59, 26.16, 25.46.


### 2.2.3. 11-Azidoundecane-1,2-Diol1b

Starting from the alkene2b (520 mg, 2.66 mmol), NMO⋅H2O (730 mg, 5.32 mmol) and OsO4 according to the general dihydroxylation procedure described above, 1b (586 mg, 96%) was obtained as
colorless oil after purification by silica gel chromatography (EE/hexane =
1/1). Rf = 0.30 (EE/hexane = 1/1).1H-NMR (200 MHz, CDCl3) δ  (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38 (m,
1H), 3.24 (t, 2H), 1.59 (m, 2H), 1.29 (m, 14H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 72.22, 66.60, 51.32, 32.93, 29.53, 29.32,
29.02, 28.69, 26.57,  25.50.


### 2.2.4. 11-Bromoundecane-1,2-Diol1c

11-Bromoundec-1-ene2c (1 g, 4.29 mmol), NMO⋅H2O (1.3 g, 9.43 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated along the general
dihydroxylation procedure described above, yielding product 1c as a white solid (1.12 g, 98%). Rf = 0.44 (EE/hexane = 3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38
(m, 1H), 3.40 (t, 2H), 1.81 (m, 2H), 1.41 (m, 2H), 1.29 (m, 12H). 13C-NMR
(50 MHz, CDCl3) δ (ppm)
= 72.22, 66.60, 33.97, 33.02, 32.72, 29.30, 28.66, 28.05, 25.51.


### 2.2.5. 5-(Dimethylamino)-N-(undec-10-Enyl)Naphthalene-1-Sulfonamide1d

Alkene2d (400 mg, 1 mmol), NMO⋅H2O (275 mg, 2 mmol), and OsO4 were reacted
in 20 mL dichloromethane and treated along the general dihydroxylation procedure
described above. The crude product was purified by silica gel chromatography
(EE/hexane = 3/1) yielding in 1d as
yellow green oil (410 mg, 94%). Rf = 0.23 (EE/hexane =
3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 8.54 (d, 1H), 8.27 (m, 2H), 7.55 (m, 2H),
7.19 (d, 1H), 4.71 (broad s, 1H), 3.63 (d, 2H), 3.42 (m, 1H), 2.89 (s, 6H),
2.04 (broad s, 2H), 1.38–1.10 (m, 18H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 134.92, 130.11, 129.70, 129.64, 129.55,
128.20, 123.56, 115.41, 72.26, 66.68, 45.56, 43.32, 33.74, 29.50, 29.26, 29.00,
28.91, 28.84, 26.37.


### 2.2.6. 5-(10,11-Dihydroxyundecanamido)-N1,n3-Bis(6-Octanamidopyridin-2-Yl)isophthalamide 1e

The alkene2e (500 mg, 0.64 mmol), NMO⋅H2O (176 mg, 1.28 mmol), and OsO4 were dissolved in 15 mL dichloromethane and treated along the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1e was obtained
as white solid (512 mg, 98%). Rf = 0.31 (EE/hexane =
4/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 9.34 (s, 1H), 8.93–7.37 (m, 9H), 3.63 (d, 2H), 3.42 (m, 1H), 3.41
(t, 2H), 2.50–2.35 (m, 6H), 2.00 (m, 2H), 1.73 (m, 2H), 1.62
(m, 4H), 1.27 (m, 24H), 0.87 (t, 6H). 13C-NMR (50 MHz, CDCl3)
δ (ppm) = 173.03, 164.72, 156.23, 149.04,
141.07, 137.63, 134.57, 120.65, 120.53, 105.83, 75.44, 70.58 36.11, 33.86,
31.83, 29.69, 29.58, 28.94, 28.65, 28.63, 25.64, 22.73, 14.11.


### 2.2.7. 4-(1-(Tert-Butyl(2-Methyl-1-Phenylpropyl)Aminooxy)Ethyl)Benzyl 10,11-Dihydroxyundecanoate1f

The alkene2f [37] (450 mg, 0.86 mmol), NMO⋅H2O (237 mg, 1.73 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1f was obtained
as colorless oil (461 mg, 96%). Rf = 0.46 (CHCl3/MeOH
= 20/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 7.42–7.15 (m, 18H), 4.12–3.16 (m, 10),
2.33 (t, J = 7.5 Hz, 4H), 1.63–1.32 (m, 15H), 2.00 (q, 4H), 1.55 (m, 11H), 1.25
(s, 16H), 1.05 (s, 9H), 0.96 (d, 3J = 6.2 Hz, 3H), 0.77 (s, 9H), 0.55
(d, 3J = 6.4 Hz, 3H), 0.22 (d, 3J = 6.4 Hz, 3H). 13C-NMR
(50 MHz, CDCl3) δ (ppm) = 169.08, 146.03, 145.31, 142.22,
142.11, 140.04, 136.44, 135.69, 130,82, 127.56, 126.73, 125.65, 121.37, 121.09,
119.13, 109.52, 108.77, 82.59, 81.68, 71.53, 66.48, 36.89, 35.08, 31.46, 31.17,
28.27, 27.86, 24.91, 24.22, 22.58, 22.08, 21.59, 20.55, 20.46.


### 2.2.8. Preparation of Superparamagnetic Iron-Oxide NanoparticlesNP1

Preparation of
octylamine-covered nanoparticles was accomplished according to literature
[35, 36]. In brief, a solution of iron cupferron
complex (780 mg, 1.5 mmol) in octylamine (3.90 g, 27.22 mmol) was alternately evacuated to
10 mbar and aerated with Ar at 60°C. Trioctylamine (7.00 g, 18.33 mmol) was
treated the same way at 100°C. The degassed octylamine solution was rapidly
injected into the trioctylamine solution at 300°C. Immediately after the
injection, the reaction vessel was moved to a 225°C prewarmed bath and
remained there for 30 minutes under vigorous magnetic stirring. After the reaction
was stopped by removing the heating source, the particles were allowed to cool
to room temperature. They were dissolved in anhydrous toluene, precipitated by
the addition of dry methanol, centrifuged, and again dissolved in anhydrous
toluene. This washing step was repeated three times. The stabilized particles
were stored under Ar-atmosphere in anhydrous toluene. Nanoparticle sizes were
determined by DLS and TEM measurements. The stabilized NPs (NP1) stay soluble for several months,
indicating that no agglomeration takes place. IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2926 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1623 (−NH2), 1464 (−CH3, −CH2−).


### 2.2.9. Ligand Exchange ofNP1 Leading to NP3a–NP3f

Surface
modification reactions were performed according to the general protocol for ligand exchange
reactions of Boal et al. [35, 36]. All
reaction steps were performed under Ar-atmosphere. 50 mg stabilized NPs (NP1) were dissolved in 30 mL of a 1:1
mixture of CHCl3 and toluene. A second mixture was prepared
consisting of 250 mg of diol-ligand 1a–f in a 1:1-mixture of chloroform and toluene. If the solubility of the ligand in
this mixture was too poor—which occurred
using ligands 1a, d, e—the smallest
amount of anhydrous isopropanol was added to dissolve the ligand. The ligand
solution was added to the nanoparticle solution via a syringe and the reaction
mixture was heated up to 50°C under vigorous stirring for 2 days. After ~48
hours, the exchange reaction was finished, resulting in surface modified
nanoparticles NP3a–f. The particles
were purified by repeated precipitation with dry methanol, centrifugation, and
resuspension in anhydrous toluene. Ligand exchange was monitored by FTIR measurements, analyzing the dried
modified nanoparticles as KBr-pellets.NP3a: IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2928 (−CH3, −CH2−), 2856 (−CH3,
−CH2−), 1702 (O=C–NH–C=O, six membered ring), 1450 (−CH3,
−CH2−), 1300 (−OH). NP3b:
IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 2094 (−N3), 1450 (−CH3, −CH2−). NP3c: IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2924 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1469 (−CH3, −CH2−), 775 (−CH2-Br). NP3d: IR (KBr-pellet) ν (cm-1)
3433 (−OH), 2956 (−CH3, −CH2−), 2916 (−CH3,
−CH2−), 2857 (−CH3, −CH2−), 1450 (−CH3,
−CH2−), 1373 (−SO2–NH−),
1150 (−SO2−). NP3e: IR
(KBr-pellet) ν (cm-1)
3420 (−OH), 2951 (−CH3, −CH2−), 2917 (−CH3,
−CH2−), 1628 (O=C–NR2), 1520 (O=C–NR2), 1450
(−CH3, −CH2−). NP3f:
IR (KBr-pellet) ν (cm-1)
3431 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 1740 (−COOR), 1456 (−N–O−), 1260 (−OH), 1030 (2 signals for
esters).


## 2.2.1. Preparation of the 1,2-Diol-Ligands1a–1f by Osmium-Tetroxide Catalyzed Dihydroxylation Reaction of Terminal Olefines 2a–2f

The
corresponding olefin was suspended in the denoted amount of Ar-bubbled solvent.
NMO⋅H2O (N-methylmorpholine-N-oxide monohydrate) was added in the
specified amount. The solution was stirred at room temperature until all
reagents were dissolved. During agitation, the solution was bubbled with argon.
Now catalytic amounts (one granule) of OsO4 were added in Ar counterflow. The
solution turned immediately yellow after the addition. The reaction mixture was
stirred  at room temperature until TLC indicates full conversion, which in all
cases occurred after 1 hour. Since OsO4 is a very strong oxidizing
agent it has to be decomposed before workup. Therefore 50 mL of an aqueous 10 w% NaHSO3 solution were added. The reaction mixture turned violet and a precipitate
occurred, which was filtered. Phases were transferred into a separation funnel
and separated. The organic layer was washed twice with water and dried with
brine and Na2SO4. Solvents were removed under reduced
pressure.


## 2.2.2. Data for the Ligands1a–1f

5-(10,11-Dihydroxyundecyl)-5-Ethylpyrimidine-2,4,6-Trione1a
The alkene2a (3 g, 9.73 mmol), NMO⋅H2O (3 g, 21.90 mmol), and OsO4 were
suspended in 50 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. The crude product was purified by
silica gel chromatography (EE/hexane = 3/1) yielding in 1a as a colorless oil (3 g, 90%). Rf = 0.48 (EE/hexane = 3/1). 1H-NMR (200 MHz, DMSO-d6) δ (ppm) = 11.57 (s, 2H), 4.40 (m, 2H), 3.26 (m,
2H), 1.83 (t, 4H) 1.39–1.23 (m, 16H), 0.77 (t, 3H). 13C-NMR (50 MHz, DMSO-d6) δ (ppm) = 174.12, 150.86, 72.06, 66.97, 56.71,
38.91, 34.38, 32.66, 30.26, 30.02, 29.84, 29.59, 26.16, 25.46.


## 2.2.3. 11-Azidoundecane-1,2-Diol1b

Starting from the alkene2b (520 mg, 2.66 mmol), NMO⋅H2O (730 mg, 5.32 mmol) and OsO4 according to the general dihydroxylation procedure described above, 1b (586 mg, 96%) was obtained as
colorless oil after purification by silica gel chromatography (EE/hexane =
1/1). Rf = 0.30 (EE/hexane = 1/1).1H-NMR (200 MHz, CDCl3) δ  (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38 (m,
1H), 3.24 (t, 2H), 1.59 (m, 2H), 1.29 (m, 14H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 72.22, 66.60, 51.32, 32.93, 29.53, 29.32,
29.02, 28.69, 26.57,  25.50.


## 2.2.4. 11-Bromoundecane-1,2-Diol1c

11-Bromoundec-1-ene2c (1 g, 4.29 mmol), NMO⋅H2O (1.3 g, 9.43 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated along the general
dihydroxylation procedure described above, yielding product 1c as a white solid (1.12 g, 98%). Rf = 0.44 (EE/hexane = 3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 4.80 (broad s, 2H), 3.57 (d, 2H), 3.38
(m, 1H), 3.40 (t, 2H), 1.81 (m, 2H), 1.41 (m, 2H), 1.29 (m, 12H). 13C-NMR
(50 MHz, CDCl3) δ (ppm)
= 72.22, 66.60, 33.97, 33.02, 32.72, 29.30, 28.66, 28.05, 25.51.


## 2.2.5. 5-(Dimethylamino)-N-(undec-10-Enyl)Naphthalene-1-Sulfonamide1d

Alkene2d (400 mg, 1 mmol), NMO⋅H2O (275 mg, 2 mmol), and OsO4 were reacted
in 20 mL dichloromethane and treated along the general dihydroxylation procedure
described above. The crude product was purified by silica gel chromatography
(EE/hexane = 3/1) yielding in 1d as
yellow green oil (410 mg, 94%). Rf = 0.23 (EE/hexane =
3/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 8.54 (d, 1H), 8.27 (m, 2H), 7.55 (m, 2H),
7.19 (d, 1H), 4.71 (broad s, 1H), 3.63 (d, 2H), 3.42 (m, 1H), 2.89 (s, 6H),
2.04 (broad s, 2H), 1.38–1.10 (m, 18H). 13C-NMR (50 MHz,
CDCl3) δ (ppm) = 134.92, 130.11, 129.70, 129.64, 129.55,
128.20, 123.56, 115.41, 72.26, 66.68, 45.56, 43.32, 33.74, 29.50, 29.26, 29.00,
28.91, 28.84, 26.37.


## 2.2.6. 5-(10,11-Dihydroxyundecanamido)-N1,n3-Bis(6-Octanamidopyridin-2-Yl)isophthalamide 1e

The alkene2e (500 mg, 0.64 mmol), NMO⋅H2O (176 mg, 1.28 mmol), and OsO4 were dissolved in 15 mL dichloromethane and treated along the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1e was obtained
as white solid (512 mg, 98%). Rf = 0.31 (EE/hexane =
4/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 9.34 (s, 1H), 8.93–7.37 (m, 9H), 3.63 (d, 2H), 3.42 (m, 1H), 3.41
(t, 2H), 2.50–2.35 (m, 6H), 2.00 (m, 2H), 1.73 (m, 2H), 1.62
(m, 4H), 1.27 (m, 24H), 0.87 (t, 6H). 13C-NMR (50 MHz, CDCl3)
δ (ppm) = 173.03, 164.72, 156.23, 149.04,
141.07, 137.63, 134.57, 120.65, 120.53, 105.83, 75.44, 70.58 36.11, 33.86,
31.83, 29.69, 29.58, 28.94, 28.65, 28.63, 25.64, 22.73, 14.11.


## 2.2.7. 4-(1-(Tert-Butyl(2-Methyl-1-Phenylpropyl)Aminooxy)Ethyl)Benzyl 10,11-Dihydroxyundecanoate1f

The alkene2f [37] (450 mg, 0.86 mmol), NMO⋅H2O (237 mg, 1.73 mmol), and OsO4 were dissolved in 20 mL dichloromethane and treated according to the general
dihydroxylation procedure described above. No further purification was
necessary. Product 1f was obtained
as colorless oil (461 mg, 96%). Rf = 0.46 (CHCl3/MeOH
= 20/1). 1H-NMR (200 MHz, CDCl3) δ (ppm) = 7.42–7.15 (m, 18H), 4.12–3.16 (m, 10),
2.33 (t, J = 7.5 Hz, 4H), 1.63–1.32 (m, 15H), 2.00 (q, 4H), 1.55 (m, 11H), 1.25
(s, 16H), 1.05 (s, 9H), 0.96 (d, 3J = 6.2 Hz, 3H), 0.77 (s, 9H), 0.55
(d, 3J = 6.4 Hz, 3H), 0.22 (d, 3J = 6.4 Hz, 3H). 13C-NMR
(50 MHz, CDCl3) δ (ppm) = 169.08, 146.03, 145.31, 142.22,
142.11, 140.04, 136.44, 135.69, 130,82, 127.56, 126.73, 125.65, 121.37, 121.09,
119.13, 109.52, 108.77, 82.59, 81.68, 71.53, 66.48, 36.89, 35.08, 31.46, 31.17,
28.27, 27.86, 24.91, 24.22, 22.58, 22.08, 21.59, 20.55, 20.46.


## 2.2.8. Preparation of Superparamagnetic Iron-Oxide NanoparticlesNP1

Preparation of
octylamine-covered nanoparticles was accomplished according to literature
[35, 36]. In brief, a solution of iron cupferron
complex (780 mg, 1.5 mmol) in octylamine (3.90 g, 27.22 mmol) was alternately evacuated to
10 mbar and aerated with Ar at 60°C. Trioctylamine (7.00 g, 18.33 mmol) was
treated the same way at 100°C. The degassed octylamine solution was rapidly
injected into the trioctylamine solution at 300°C. Immediately after the
injection, the reaction vessel was moved to a 225°C prewarmed bath and
remained there for 30 minutes under vigorous magnetic stirring. After the reaction
was stopped by removing the heating source, the particles were allowed to cool
to room temperature. They were dissolved in anhydrous toluene, precipitated by
the addition of dry methanol, centrifuged, and again dissolved in anhydrous
toluene. This washing step was repeated three times. The stabilized particles
were stored under Ar-atmosphere in anhydrous toluene. Nanoparticle sizes were
determined by DLS and TEM measurements. The stabilized NPs (NP1) stay soluble for several months,
indicating that no agglomeration takes place. IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2926 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1623 (−NH2), 1464 (−CH3, −CH2−).


## 2.2.9. Ligand Exchange ofNP1 Leading to NP3a–NP3f

Surface
modification reactions were performed according to the general protocol for ligand exchange
reactions of Boal et al. [35, 36]. All
reaction steps were performed under Ar-atmosphere. 50 mg stabilized NPs (NP1) were dissolved in 30 mL of a 1:1
mixture of CHCl3 and toluene. A second mixture was prepared
consisting of 250 mg of diol-ligand 1a–f in a 1:1-mixture of chloroform and toluene. If the solubility of the ligand in
this mixture was too poor—which occurred
using ligands 1a, d, e—the smallest
amount of anhydrous isopropanol was added to dissolve the ligand. The ligand
solution was added to the nanoparticle solution via a syringe and the reaction
mixture was heated up to 50°C under vigorous stirring for 2 days. After ~48
hours, the exchange reaction was finished, resulting in surface modified
nanoparticles NP3a–f. The particles
were purified by repeated precipitation with dry methanol, centrifugation, and
resuspension in anhydrous toluene. Ligand exchange was monitored by FTIR measurements, analyzing the dried
modified nanoparticles as KBr-pellets.NP3a: IR (KBr-pellet) ν (cm-1)
3450 (−OH), 2928 (−CH3, −CH2−), 2856 (−CH3,
−CH2−), 1702 (O=C–NH–C=O, six membered ring), 1450 (−CH3,
−CH2−), 1300 (−OH). NP3b:
IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 2094 (−N3), 1450 (−CH3, −CH2−). NP3c: IR (KBr-pellet) ν (cm-1)
3420 (−OH), 2924 (−CH3, −CH2−), 2857 (−CH3,
−CH2−), 1469 (−CH3, −CH2−), 775 (−CH2-Br). NP3d: IR (KBr-pellet) ν (cm-1)
3433 (−OH), 2956 (−CH3, −CH2−), 2916 (−CH3,
−CH2−), 2857 (−CH3, −CH2−), 1450 (−CH3,
−CH2−), 1373 (−SO2–NH−),
1150 (−SO2−). NP3e: IR
(KBr-pellet) ν (cm-1)
3420 (−OH), 2951 (−CH3, −CH2−), 2917 (−CH3,
−CH2−), 1628 (O=C–NR2), 1520 (O=C–NR2), 1450
(−CH3, −CH2−). NP3f:
IR (KBr-pellet) ν (cm-1)
3431 (−OH), 2920 (−CH3, −CH2−), 2848 (−CH3,
−CH2−), 1740 (−COOR), 1456 (−N–O−), 1260 (−OH), 1030 (2 signals for
esters).


## 3. Results and Discussion

The basic strategy for the surface modification of
the iron oxide nanoparticles with various organic ligands is shown in Figure1.
Starting from octylamine-covered Fe2O3 nanoparticles, a
direct ligand exchange reaction with substituted 1,2-diols in organic solvents
takes place, using the 1,2-diol-moiety as attachment site for the NP-binding. Critical
factors, such as the kinetics of ligand exchange, the quantification of surface
coverage are described. Basically, four different types of ligands are
described: (a) the supramolecular ligands 1a and 1e, (b) ligands bearing the
fluorescent label 1d, (c) the azido-
and bromine ligands 1b and 1c, as well as (d) the polymerization
initiator 1f.


### 3.1. Preparation of 1,2-Diol-Ligands1a–1f

The
functionalization ofγ-Fe2O3 nanoparticles with
different targeting ligands asks for the creation of an effective nanoparticle binding site
on the corresponding ligand. Therefore, the synthesis of 1,2-diols (1a–1f) (see Scheme 1) was planned and
prepared via a non-stereoselective Sharpless dihydroxylation [38].
The reaction was effected by the addition of catalytic amounts of OsO4 to a mixture of N-methyl-morpholine-N-oxide (NMO) and the corresponding
(terminal) alkenes 2a–2f in CH2Cl2. Using this methodology, the ligands 1a–1f were prepared in 85–98% yield. 1H-NMR spectra and 13C-NMR-spectra
are indicative of the high purity and chemical integrity of the prepared
samples.Scheme 1
Synthesis of the
1,2-diol-ligands1a–1f via dihydroxylation of the corresponding terminal alkenes 2a–2f.


### 3.2. Preparation of Iron Oxide Nanoparticles

Iron oxide  nanoparticles were
prepared by use of the thermal decomposition of iron cupferron complex in octylamine and
trioctylamine (see Scheme 2) at elevated temperatures, as described by Alivisatos
[35] and Boal et al. [36], yielding NP1 (r = 3.6–4.2 nm, as determined by DLS-measurements)
with an octylamine shell
around the iron oxide nanoparticles. The stabilized nanoparticles are soluble
for several month, indicating the absence of agglomeration effects.The  nanoparticlesNP1 were characterized by UV-VIS and
fluorescence-spectroscopy (see supplementary available online at doi:10.1155/2008/383020) as well as via
TEM, XRD, and IR-spectroscopy (see Figure 2).Data for the octylamine-covered nanoparticlesNP1 (a) TEM micrograph, (b) XRD of NP1, (c) FT-IR of NP1.
(a)(b)(c)TEM measurements (see Figure 2(a)) were
performed to prove the size of the synthesized nanoparticles and to determine
the shape of the NPs. According to these measurements, γ-Fe2O3 nanoparticles
crystallize in a spherical shape with a diameter of 6–15 nm which is in
good accordance with DLS results.
The determination of the lattice structure of the synthesized nanoparticles was
accomplished by powder XRD measurements (see Figure 2(b)). Comparing the resulting reflexes to the
patterns of both possible structures revealed that the synthesized
nanoparticles crystallize in the γ-Fe2O3-structure. Due to the small size of the nanoparticle
in comparison to the bulk material, the reflexes
are not as sharp but the angle-2Θ-values fit to the
bulk pattern. FTIR spectroscopy
was used to determine the chemical nature of the surface bound ligands, for
example, octylamine (see Figure 2(c)). The nanoparticle spectrum (bottom) is
showing all peaks at the characteristic frequencies, such as 2926 cm-1,
2857 cm-1 (−CH), and 1623 cm-1 (−NH2), but with
a slight line broadening and less intensity. This is in good agreement with
literature [36], showing the
same phenomenon.An important parameter
concerns the surface coverage of the nanoparticles with the organic ligand
(octylamine).In order to generate a
value of significance, both a theoretical calculation and an experimental
determination via TGA analysis were performed (see Figures3(a) and 3(b)). According to
the literature [36], γ-Fe2O3 crystallizes in a cubic closed-packed
structure. In this sphere, each atom has 6 neighbors in plane and 3 in each
plane above and below, leading to a sum of 12 neighbor atoms. Referring to DLS measurements, NP1 has an average radius of 3.92 nm, leading to  a volume of the particle of Vparticle = 252.32 nm3 and a particle surface of Sparticle = 193.10 nm2.
Considering the density of γ-Fe2O3 (ρFe2O3 = 4.9 g/cm3),
the theoretical mass of one particle mparticle = 1.24⋅10-18 g.
Taking the surface of one particle (Sparticle) and the binding area
of one ligand (Aoctylamine) on the nanoparticle surface into
account, the amount of surface bound ligands (noctylamine) can be
calculated. The binding area of one ligand is assumed as quadratic, therefore,
Aoctylamine = (aoctylamine)2, where aoctylamine is the side length of the quadratic binding area of one ligand. Using the
molecular weight of the surface bound ligand (Moctylamine = 129.3 g/mol),
it is now possible to determine the theoretical mass fraction of the surface
bound ligands with respect to the total mass of the nanoparticle. Furthermore,
paying attention to the molar mass of γ-Fe2O3 (MFe2O3 = 159.7 g/mol)
and to the length of the elemental cell of γ-Fe2O3 (aFe2O3 = 0.8339 nm),
it is possible to determine the number of γ-Fe2O3-units per
nanoparticle (nFe2O3) and in the outer shell of the nanoparticle (nouterFe2O3).(a) TGA-graph ofNP1, (b) schematic flow scheme for
the calculation of the theoretical coverage of NP1 with n-octylamine as ligand, (c) geometries of shells and shell
thicknesses.
(a)(b)(c)TGA (thermogravimetric analysis)
measurements of NP1 were performed
to estimate the amount of surface bound ligands. The mass loss of 22.02 w%
indicates that the total mass of a nanoparticle NP1 is composed of 78 w% iron oxide core and 22 w% organic stabilizing ligand, thus being in excellent accordance
to the calculated value. Therefore, TGA was assumed to represent an excellent
tool for the characterization of the amount of surface bound ligand.


### 3.3. Ligand Exchange with 1,2-Diols

Using the
general protocol for ligand exchange reactions according to  Boal et al. [36], stabilized NP1 are
treated with a 5-fold excess of diol-ligand 1a–f in a 1:1-mixture of CHCl3 and toluene at 50°C. The
reaction finished after 48 hours resulting in surface modified nanoparticles NP3a–f. The particle size was
determined by DLS measurements showing no change in particle diameter.A first hint
at a successful ligand exchange was provided by solubility experiments. Thus the
solubility ofNP3a–f was rather
different as compared to the nonmodified nanoparticle (NP1a–d). Whereas NP3a, c, f were completely soluble in
common organic nonpolar aprotic solvents, such as CHCl3 and toluene, NP3a, d, e were only poorly soluble in
any of these solvents, presumably due to self aggregation phenomena of the
respective multiple hydrogen bonding interaction, now present at their surface
due to the ligand exchange reaction. Even ultrasonic irradiation was not able
to improve their solubility. NP3a were soluble in dipolar aprotic solvents such as in DMF or THF. This behavior
is also visible in TEM measurements, where the formation of aggregates of
nanoparticles NP3a is clearly
visible (see Figure 4(a)) in comparison to the noninteracting NP3c. XRD measurement still indicates
the presence of γ-Fe2O3 after the exchange
reaction (see Figure 4(b)).(a) TEM micrographs ofNP3a and NP3c, (b) XRD spectrum of NP3a.
(a)(b)In order to fully
characterize the surface bound ligands onNP3a–f, FTIR measurements were performed. Figures 5(a)–5(f) show the IR spectra of all synthesized nanoparticles,
in each case the upper spectrum shows the unbound ligand, the lower one of the
functionalized NPs. Since the binding site of the ligand on the nanoparticle  is
the electron donating diol group, an OH-signal is always present at ~3400 cm-1.
Furthermore, characteristic peaks are always present, thus proving the presence
of the ligands on the NP-surface. NP3a  show a characteristic carbonyl signal at 1702 cm-1, NP3b  show a significant azide band at 2094 cm-1.
Furthermore, NP3c  show a typical band at
1470 cm-1 and in NP3d the
band at 1373 cm-1 for the  sulfonamide moiety  is clearly visible. NP3e show
a significant band at 1628 cm-1 for the amide and in NP3f a signal at 1456 cm-1 is
indicative for the nitroxide moiety.IR spectra ofNP’s 3a–3f. In each spectrum the
upper traces are those of the free ligand, the lower traces those of the
corresponding NP-bound ligand. (a) NP3a, (b) NP3b, (c) NP3c, (d) NP3d, (e) NP3e, (f) NP3f.
(a)(b)(c)(d)(e)(f)Again, as already demonstrated before,TGA (see Figure 6) and DLS results were used for the
determination of the number of surface bound ligands, the number of iron centers
on the surface, and the ligand binding area. The following nanoparticles were
analyzed: NP3a–d and NP3f. The results are compiled in the
following table (Table 1). Comparison of the weight loss gained by TGA measurements and calculations based
on the hydrodynamic radius gained by DLS shows an excellent match, thus proving the complete ligand exchange.Table 1
TGA data and
calculation parameters forNP3a–NP3d, NP3f.
ParameterNP3aNP3bNP3cNP3dNP3frparticle3.92 nm3.59 nm5.15 nm3.92 nm8.61 nmVparticle252.32 nm3193.81 nm3572.15 nm3252. nm32668.95 nm3Sparticle193.10 nm2161.96 nm2333.29 nm2193.10 nm2930.49 nm2mparticle1.24⋅10-18 g9.50⋅10-19 g2.80⋅10-18 g1.24⋅10-18 g1.31⋅10-17 galigand0.75 nm0.29 nm0.40 nm0.75 nm0.26 nmnligand3431926208334314888nligand TGA3721946209833215336niron centers15921307289915928662nouterFe2O3238719614349238712993nFe2O34662358110572466249317CALCmass loss13.64 w%43.54 w%24.78 w%16.76 w%51.20 w%TGAmass loss14.60 w%43.80 w%24.91 w%16.29 w%51.94 w%Figure 6
TGA curves for
the surface-modified nanoparticlesNP3a–NP3d and NP3f. (data: NP3a (14.60%), NP3b (43.80%), NP3c (24.91%), NP3d (16.29%), and NP3f (51.94%).


### 3.4. Kinetics of the Ligand Exchange Reaction

This is an important point concerning the follow-up of the ligand
exchange  kinetics
during the incubation of the octylamine-covered nanoparticles  with the 1,2-diol-ligands.
As described by  Boal et al.  [36]. a time of 48 hours is required for
all diol-ligands to effect the efficient exchange reaction, replacing the
intitial octylamine ligand by the much harder  alcohol. However, ligands bearing monovalent
hydroxyl moieties are not stable over a long term, leading to irreversible
aggregation of the nanoparticles after the eventual loss of the covering
ligand.We therefore have studied the
kinetics of the ligand exchange reaction ofNP1 with ligands 1a and 1d (see Figure 7(a)), enabling to
follow the ligand exchange via TGA due to the significant increase in weight
when compared to the octylamine ligand in NP1. The  nanoparticles NP1 were  treated
under standard ligand exchange conditions with the corresponding diol-ligand (1a and 1d), respectively. Samples of ~5 mg were taken at different time
points (t1⋯tn), purified by repeated precipitation, and analyzed
thermogravimetrically. The weight loss was plotted versus time resulting  in the results in Figure 7(b),
indicating a saturation law. The final theoretical mass loss was calculated and
compared to the values gained by TGA measurements. The calculated values of 16.76 w% of ligand 1d on NP3d are in
excellent accordance with the measured value of 16.29 w% (NP3d) elicited by TGA.
In case of NP3a, the theoretically calculated mass
loss of 13.64 w% fits well the measured value of 14,60 w% determined by TGA. In comparision to the results obtained by
parallel FTIR measurements (data not shown), TGA results indicate a complete exchange of octylamine after 24
hours whereas IR results indicate a
reaction time of 48 hours.Ligand exchange experiments monitored
via TGA. (a) Scheme for the exchange ofNP1 with the ligands 1a and 1d, (b) plot of the TGA mass loss
versus time.
(a)(b)Therefore, it can be
concluded that a complete ligand exchange reaction can be effected within one
day of reaction time using a nonpolar reaction medium.


## 3.1. Preparation of 1,2-Diol-Ligands1a–1f

The
functionalization ofγ-Fe2O3 nanoparticles with
different targeting ligands asks for the creation of an effective nanoparticle binding site
on the corresponding ligand. Therefore, the synthesis of 1,2-diols (1a–1f) (see Scheme 1) was planned and
prepared via a non-stereoselective Sharpless dihydroxylation [38].
The reaction was effected by the addition of catalytic amounts of OsO4 to a mixture of N-methyl-morpholine-N-oxide (NMO) and the corresponding
(terminal) alkenes 2a–2f in CH2Cl2. Using this methodology, the ligands 1a–1f were prepared in 85–98% yield. 1H-NMR spectra and 13C-NMR-spectra
are indicative of the high purity and chemical integrity of the prepared
samples.Scheme 1
Synthesis of the
1,2-diol-ligands1a–1f via dihydroxylation of the corresponding terminal alkenes 2a–2f.


## 3.2. Preparation of Iron Oxide Nanoparticles

Iron oxide  nanoparticles were
prepared by use of the thermal decomposition of iron cupferron complex in octylamine and
trioctylamine (see Scheme 2) at elevated temperatures, as described by Alivisatos
[35] and Boal et al. [36], yielding NP1 (r = 3.6–4.2 nm, as determined by DLS-measurements)
with an octylamine shell
around the iron oxide nanoparticles. The stabilized nanoparticles are soluble
for several month, indicating the absence of agglomeration effects.The  nanoparticlesNP1 were characterized by UV-VIS and
fluorescence-spectroscopy (see supplementary available online at doi:10.1155/2008/383020) as well as via
TEM, XRD, and IR-spectroscopy (see Figure 2).Data for the octylamine-covered nanoparticlesNP1 (a) TEM micrograph, (b) XRD of NP1, (c) FT-IR of NP1.
(a)(b)(c)TEM measurements (see Figure 2(a)) were
performed to prove the size of the synthesized nanoparticles and to determine
the shape of the NPs. According to these measurements, γ-Fe2O3 nanoparticles
crystallize in a spherical shape with a diameter of 6–15 nm which is in
good accordance with DLS results.
The determination of the lattice structure of the synthesized nanoparticles was
accomplished by powder XRD measurements (see Figure 2(b)). Comparing the resulting reflexes to the
patterns of both possible structures revealed that the synthesized
nanoparticles crystallize in the γ-Fe2O3-structure. Due to the small size of the nanoparticle
in comparison to the bulk material, the reflexes
are not as sharp but the angle-2Θ-values fit to the
bulk pattern. FTIR spectroscopy
was used to determine the chemical nature of the surface bound ligands, for
example, octylamine (see Figure 2(c)). The nanoparticle spectrum (bottom) is
showing all peaks at the characteristic frequencies, such as 2926 cm-1,
2857 cm-1 (−CH), and 1623 cm-1 (−NH2), but with
a slight line broadening and less intensity. This is in good agreement with
literature [36], showing the
same phenomenon.An important parameter
concerns the surface coverage of the nanoparticles with the organic ligand
(octylamine).In order to generate a
value of significance, both a theoretical calculation and an experimental
determination via TGA analysis were performed (see Figures3(a) and 3(b)). According to
the literature [36], γ-Fe2O3 crystallizes in a cubic closed-packed
structure. In this sphere, each atom has 6 neighbors in plane and 3 in each
plane above and below, leading to a sum of 12 neighbor atoms. Referring to DLS measurements, NP1 has an average radius of 3.92 nm, leading to  a volume of the particle of Vparticle = 252.32 nm3 and a particle surface of Sparticle = 193.10 nm2.
Considering the density of γ-Fe2O3 (ρFe2O3 = 4.9 g/cm3),
the theoretical mass of one particle mparticle = 1.24⋅10-18 g.
Taking the surface of one particle (Sparticle) and the binding area
of one ligand (Aoctylamine) on the nanoparticle surface into
account, the amount of surface bound ligands (noctylamine) can be
calculated. The binding area of one ligand is assumed as quadratic, therefore,
Aoctylamine = (aoctylamine)2, where aoctylamine is the side length of the quadratic binding area of one ligand. Using the
molecular weight of the surface bound ligand (Moctylamine = 129.3 g/mol),
it is now possible to determine the theoretical mass fraction of the surface
bound ligands with respect to the total mass of the nanoparticle. Furthermore,
paying attention to the molar mass of γ-Fe2O3 (MFe2O3 = 159.7 g/mol)
and to the length of the elemental cell of γ-Fe2O3 (aFe2O3 = 0.8339 nm),
it is possible to determine the number of γ-Fe2O3-units per
nanoparticle (nFe2O3) and in the outer shell of the nanoparticle (nouterFe2O3).(a) TGA-graph ofNP1, (b) schematic flow scheme for
the calculation of the theoretical coverage of NP1 with n-octylamine as ligand, (c) geometries of shells and shell
thicknesses.
(a)(b)(c)TGA (thermogravimetric analysis)
measurements of NP1 were performed
to estimate the amount of surface bound ligands. The mass loss of 22.02 w%
indicates that the total mass of a nanoparticle NP1 is composed of 78 w% iron oxide core and 22 w% organic stabilizing ligand, thus being in excellent accordance
to the calculated value. Therefore, TGA was assumed to represent an excellent
tool for the characterization of the amount of surface bound ligand.


## 3.3. Ligand Exchange with 1,2-Diols

Using the
general protocol for ligand exchange reactions according to  Boal et al. [36], stabilized NP1 are
treated with a 5-fold excess of diol-ligand 1a–f in a 1:1-mixture of CHCl3 and toluene at 50°C. The
reaction finished after 48 hours resulting in surface modified nanoparticles NP3a–f. The particle size was
determined by DLS measurements showing no change in particle diameter.A first hint
at a successful ligand exchange was provided by solubility experiments. Thus the
solubility ofNP3a–f was rather
different as compared to the nonmodified nanoparticle (NP1a–d). Whereas NP3a, c, f were completely soluble in
common organic nonpolar aprotic solvents, such as CHCl3 and toluene, NP3a, d, e were only poorly soluble in
any of these solvents, presumably due to self aggregation phenomena of the
respective multiple hydrogen bonding interaction, now present at their surface
due to the ligand exchange reaction. Even ultrasonic irradiation was not able
to improve their solubility. NP3a were soluble in dipolar aprotic solvents such as in DMF or THF. This behavior
is also visible in TEM measurements, where the formation of aggregates of
nanoparticles NP3a is clearly
visible (see Figure 4(a)) in comparison to the noninteracting NP3c. XRD measurement still indicates
the presence of γ-Fe2O3 after the exchange
reaction (see Figure 4(b)).(a) TEM micrographs ofNP3a and NP3c, (b) XRD spectrum of NP3a.
(a)(b)In order to fully
characterize the surface bound ligands onNP3a–f, FTIR measurements were performed. Figures 5(a)–5(f) show the IR spectra of all synthesized nanoparticles,
in each case the upper spectrum shows the unbound ligand, the lower one of the
functionalized NPs. Since the binding site of the ligand on the nanoparticle  is
the electron donating diol group, an OH-signal is always present at ~3400 cm-1.
Furthermore, characteristic peaks are always present, thus proving the presence
of the ligands on the NP-surface. NP3a  show a characteristic carbonyl signal at 1702 cm-1, NP3b  show a significant azide band at 2094 cm-1.
Furthermore, NP3c  show a typical band at
1470 cm-1 and in NP3d the
band at 1373 cm-1 for the  sulfonamide moiety  is clearly visible. NP3e show
a significant band at 1628 cm-1 for the amide and in NP3f a signal at 1456 cm-1 is
indicative for the nitroxide moiety.IR spectra ofNP’s 3a–3f. In each spectrum the
upper traces are those of the free ligand, the lower traces those of the
corresponding NP-bound ligand. (a) NP3a, (b) NP3b, (c) NP3c, (d) NP3d, (e) NP3e, (f) NP3f.
(a)(b)(c)(d)(e)(f)Again, as already demonstrated before,TGA (see Figure 6) and DLS results were used for the
determination of the number of surface bound ligands, the number of iron centers
on the surface, and the ligand binding area. The following nanoparticles were
analyzed: NP3a–d and NP3f. The results are compiled in the
following table (Table 1). Comparison of the weight loss gained by TGA measurements and calculations based
on the hydrodynamic radius gained by DLS shows an excellent match, thus proving the complete ligand exchange.Table 1
TGA data and
calculation parameters forNP3a–NP3d, NP3f.
ParameterNP3aNP3bNP3cNP3dNP3frparticle3.92 nm3.59 nm5.15 nm3.92 nm8.61 nmVparticle252.32 nm3193.81 nm3572.15 nm3252. nm32668.95 nm3Sparticle193.10 nm2161.96 nm2333.29 nm2193.10 nm2930.49 nm2mparticle1.24⋅10-18 g9.50⋅10-19 g2.80⋅10-18 g1.24⋅10-18 g1.31⋅10-17 galigand0.75 nm0.29 nm0.40 nm0.75 nm0.26 nmnligand3431926208334314888nligand TGA3721946209833215336niron centers15921307289915928662nouterFe2O3238719614349238712993nFe2O34662358110572466249317CALCmass loss13.64 w%43.54 w%24.78 w%16.76 w%51.20 w%TGAmass loss14.60 w%43.80 w%24.91 w%16.29 w%51.94 w%Figure 6
TGA curves for
the surface-modified nanoparticlesNP3a–NP3d and NP3f. (data: NP3a (14.60%), NP3b (43.80%), NP3c (24.91%), NP3d (16.29%), and NP3f (51.94%).


## 3.4. Kinetics of the Ligand Exchange Reaction

This is an important point concerning the follow-up of the ligand
exchange  kinetics
during the incubation of the octylamine-covered nanoparticles  with the 1,2-diol-ligands.
As described by  Boal et al.  [36]. a time of 48 hours is required for
all diol-ligands to effect the efficient exchange reaction, replacing the
intitial octylamine ligand by the much harder  alcohol. However, ligands bearing monovalent
hydroxyl moieties are not stable over a long term, leading to irreversible
aggregation of the nanoparticles after the eventual loss of the covering
ligand.We therefore have studied the
kinetics of the ligand exchange reaction ofNP1 with ligands 1a and 1d (see Figure 7(a)), enabling to
follow the ligand exchange via TGA due to the significant increase in weight
when compared to the octylamine ligand in NP1. The  nanoparticles NP1 were  treated
under standard ligand exchange conditions with the corresponding diol-ligand (1a and 1d), respectively. Samples of ~5 mg were taken at different time
points (t1⋯tn), purified by repeated precipitation, and analyzed
thermogravimetrically. The weight loss was plotted versus time resulting  in the results in Figure 7(b),
indicating a saturation law. The final theoretical mass loss was calculated and
compared to the values gained by TGA measurements. The calculated values of 16.76 w% of ligand 1d on NP3d are in
excellent accordance with the measured value of 16.29 w% (NP3d) elicited by TGA.
In case of NP3a, the theoretically calculated mass
loss of 13.64 w% fits well the measured value of 14,60 w% determined by TGA. In comparision to the results obtained by
parallel FTIR measurements (data not shown), TGA results indicate a complete exchange of octylamine after 24
hours whereas IR results indicate a
reaction time of 48 hours.Ligand exchange experiments monitored
via TGA. (a) Scheme for the exchange ofNP1 with the ligands 1a and 1d, (b) plot of the TGA mass loss
versus time.
(a)(b)Therefore, it can be
concluded that a complete ligand exchange reaction can be effected within one
day of reaction time using a nonpolar reaction medium.


## 4. Conclusion

In summary, we have developed
an easy access to the surface modification ofγ-Fe2O3 nanoparticles via exchange reaction with 1,2-diols. The generation of the
corresponding ligands is simple, since the 1,2-dihydroxylation reaction can be performed
in the presence of a multitude of functional groups, as demonstrated with the
ligands 1a–1f. Clearly, the method offers the
possibility to engineer superparamagnetic iron oxide nanoparticles with a large
variety of functional organic ligands, being bound stable to the nanoparticle
surface.



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*Source: 383020-2008-09-04.xml*