Sensing of Biologically Important Cations Such as Na
+
,K
+
, Ca
2+
, Cu
2+
,
and Fe
3+
Using Magnetic Nanoemulsions
V. Mahendran and John Philip*
SMARTS, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, TamilNadu, India
* S Supporting Information
ABSTRACT: We report a simple approach to the ultrasensitive detection
of biologically important metal ions using a magnetic nanoemulsion. The
nanoemulsion used in our study was an oil-in-water emulsion droplet of
average size ∼190 nm containing ferrimagnetic iron oxide nanoparticles of
average size ∼10 nm. In a static magnetic field, the emulsion droplets self-
assemble into a nanoarray with distinct interdroplet spacing. In the presence
of cations in the solution, the nanofluid array shows a large blue shift in the
diffracted Bragg peak and a visually perceivable color change due to changes
in the electrical double layer upon the diffusion of cations. The colloidal
force−distance measurements in the presence of cations show large
variations at the onset of repulsion in the presence of cations. The sensor
shows good selectivity to Na
+
,K
+
, Ca
2+
, Cu
2+
, and Fe
3+
ions and offers a
rapid response compared to conventional techniques. This approach can be
useful for the recognition of biologically important cations.
1. INTRODUCTION
An excess or deficiency of heavy metal ions such as Fe
3+
, Zn
2+
,
and Cu
2+
and intracellular ions such as Na
+
,K
+
, Ca
2+
, and Mg
2+
in human body fluids can lead to various biological disorders.
For example, Fe
3+
plays an essential role in oxygen uptake,
metabolism, and electron transfer in the body.
1
An Fe
3+
deficiency can lead to the permanent loss of motor skills, and
its excess can lead to diseases such as Parkinson’s and
Alzheimer’s.
2
Therefore, periodic monitoring of cations is a
prerequisite for studying the physiological functions and the
diagnosis of diseases and their prevention. The techniques for
the detection of ions at low concentrations use polymer
hydrogels,
3,4
conducting polymer nanoarrays,
5
core/shell
microsphere-based luminescent probes,
6
polyelectrolyte films,
7
biomediated silver nanoparticles,
8
gold nanoparticles,
9
con-
ducting thin films,
10
OTFTs (organic thin film transistors),
11
metal ion-based fluorescence,
12
luminescence,
13
electrochem-
ical methods,
14
colorimetric approaches based on the catalytic
leaching of silver-coated gold nanoparticles,
15
an allosteric dual-
DNAzyme-based method,
16
self-assembled monolayers,
17
SERS
(surface enhanced Raman scattering),
18,19
magnetic materi-
als,
6,20
holography,
21
photonic crystals,
22
1D periodic block
copolymer photonic lamellar gels,
23
and functionalized
hexagonal ZnO nanorod-based electrochemical sensors.
24
Some of these techniques are expensive, complex in design,
nonportable, and involve detailed data analysis.
10
This triggered
an interest in developing versatile, inexpensive, portable, and
easy to use techniques for the rapid and accurate detection of
toxic metal cations in our fresh water resources, food items and
body fluids.
25
Many strategies have been developed for sensing
biologically important cations such as Fe
3+
,
1,26,27
Zn
2+
,
3,28
Cu
2+
,
5,6,8,15,16,20,29−31
Na
+
,
3,21,32,33
Ca
2+
,
12,24,34
and Mg
2+
and
anions.
23,35−37
Despite the developments in this field, many
practical challenges remain in packaging these sensors to reach
the market as a result of the complex fabrication procedures
involved and the requirements of specific chemical function-
alities. Other major drawbacks of some of the existing sensors is
their slow response and poor sensitivity/selectivity.
Here, we report a simple approach suitable for the
recognition of metal ions with ultrahigh sensitivity (parts per
million level) and selectivity using stimuli-responsive, magneti-
cally polarizable oil-in-water nanoemulsion droplets. Over the
years, magnetic dispersions have evolved as a new class of
stimuli-responsive smart materials with numerous applica-
tions,
36,38
and they have also been a wonderful model system
for fundamental studies.
39−42
The new sensor has been tested
for various biologically important cations such as Na
+
,K
+
, Ca
2+
,
Cu
2+
, and Fe
3+
. Unlike other approaches, the present approach
involves neither ion receptor entities nor complex preparation
techniques. The response time of the new sensor is about 2
orders of magnitude faster than that of photonic-crystal-based
sensors. We obtain insight into the underlying mechanism for
the changes in optical properties by measuring the subtle
changes in the intermolecular forces between droplets.
2. MATERIALS AND METHODS
2.1. Materials. Sodium dodecyl sulfate
(CH
3
(CH
2
)
10
CH
2
SO
4
−
Na
+
) was purchased from Aldrich and used
Received: February 6, 2013
Revised: March 5, 2013
Published: March 11, 2013
Article
pubs.acs.org/Langmuir
© 2013 American Chemical Society 4252 dx.doi.org/10.1021/la400502b | Langmuir 2013, 29, 4252−4258