A modified mesoporous silica optical nanosensor for selective monitoring of multiple analytes in water

Article abstract of DOI:10.1039/c3cc42513d

A new modified mesoporous silica nanosensor was synthesized by the co-condensation method. Under basic conditions, the obtained mesoporous silica nanosensor responds selectively to Fe²⁺ (pH = 8) and Cu²⁺ (pH = 12) with a distinguishable colour change perceivable by the naked eye and a detection limit of approximately 50 ppb. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc42513d

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A modified mesoporous silica optical nanosensor for
selective monitoring of multiple analytes in water†
Cite this: DOI: 10.1039/c3cc42513d
Madhappan Santha Moorthy, Hun-Jeong Cho, Eun-Jeong Yu, Young-Sik Jung and
Chang-Sik Ha*
Received 7th April 2013,
Accepted 28th May 2013
DOI: 10.1039/c3cc42513d
www.rsc.org/chemcomm
A new modified mesoporous silica nanosensor was synthesized by for the simultaneous colorimetric detection of two metal cations.12
the co-condensation method. Under basic conditions, the obtained The development of mesoporous nanoparticles MCM-41 and SBA-15
mesoporous silica nanosensor responds selectively to Fe2+ (pH = 8) for solid-type nanosensors is key to designing highly sensitive
and Cu2+ (pH = 12) with a distinguishable colour change perceivable and selective heterogeneous optical sensors, which have several
by the naked eye and a detection limit of approximately 50 ppb.
advantages over the homogeneous probes; e.g. large surface area,
pore volume, hydrophilicity, thermal and mechanical stability,
Heavy metal (transition metal) ions generally play crucial roles and easy recyclability.13 In this study, we prepared a modified
in both biological systems and the evolving field of bionano- mesoporous silica nanosensor (SAPy-MMS) that allows for the
technology.1 Recent research efforts tend to focus on the selective detection of two analytes with distinct spectral responses
potential impact of transition metal ions and their toxicity on (absorption shifted) and colour changes; specifically, the colori-
human health and the environment.2 Among the first row metric response only occurs with the addition of Fe2+ (dark brown,
transition metal ions, Cu2+ and Fe2+ play important roles in
l
max 325 nm) and Cu2+ (green, lmax 365 nm), whereas other metal
extra- and intracellular functions of metabolic processes.3 On the ions do not interfere with the absorption spectra of SAPy-MMS or
other hand, the excess quantities of these metal ions can cause its ability to detect Fe2+ and Cu2+.
serious neurodegenerative conditions, like Wilson’s disease,4
In Fig. S1 (ESI†), the FTIR spectrum of SAPy-MMS shows typical
Alzheimer’s disease,5 and prion diseases.6 The development bands at 2869 and 2941 cmÀ1 attributed to C–H stretching vibra-
of materials for the detection of significant metal cations is a tions of silylated SAPy segments. The peak appearing at 1492 cmÀ1
synergistic advancement while considering both the environmental indicates the presence of the N–H bands of SAPy groups. The
and biological importance.7 Recently, the development of metal- vibration bands at 1638 and 1718 cmÀ1 are characteristic of CQN
sensitive chromogenic probes has been receiving increased atten- and CQO groups, confirming that the covalently bonded SAPy
tion because of their various potential applications, and it is crucial functional organosilane moieties are integrated into the mesoporous
to pursue more simplistic nanosensor designs to very selectively silica network. The bands at 1075 cmÀ1 and 963 cmÀ1 indicate the
detect important physiological and environmental metal cations. presence of Si–O–Si bonds and Si–OH bonds, respectively. A broad
So far, a variety of homogeneous chemosensors have been band appearing in the range 3200–3500 cmÀ1 suggests the presence
reported,8 and they can be utilized for the immediate optical of silanol groups in the silica nanosensor.14 The FTIR result
detection of various metal cations by an instant colour change that confirms the successful incorporation of SAPy groups in the
can be perceived by the naked eye. However, these homogeneous silica network (Scheme 1).
sensors have some drawbacks that critically affect their recovery/
Fig. 1A shows the nitrogen adsorption–desorption isotherm
reuse. Alternatively, recently developed heterogeneous solid-state of SAPy-MMS and the corresponding pore size distributions.
chemosensors, such as silica nanoparticles,9 magnetic silica,10 The sample displays a type IV isotherm with a H1 type hysteresis
and silica nanowires,11 are highly applicable due to their rapid loop,15 characteristic of mesoporous materials with uniform pore
detection and recyclability in practical applications. Currently, size distributions. The specific surface area (SBET), pore volume, and
very few solid-based chemosensor materials have been reported pore diameter values are listed in Table S1 (ESI†), and they indicate
ordered mesoporosity with uniform mesopore size distribution.16
Fig. S2 (ESI†) shows the XRD patterns of the as-synthesized and
surfactant extracted SAPy-MMS. Both as-synthesized and surfactant
extracted SAPy-MMS show a sharp (100) XRD reflection peak around
Department of Polymer Science and Engineering, Pusan National University,
Geumjeong-gu, Busan, 609-735, Korea. E-mail: csha@pnu.edu;
Fax: +82-51-514-4331; Tel: +82-51-510-2407
2y = 2.381 with a d-spacing value of 3.8 nm, while the peak intensity
of (100) reflection becomes sharper and the additional (110), (200)
† Electronic supplementary information (ESI) available: Experimental details and
supporting figures. See DOI: 10.1039/c3cc42513d
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also be observed (albeit with weak intensities), which can be
attributed to CH2, CQN, and CQO, respectively.18 From the
elemental analysis of carbon and nitrogen content, the amount
of loaded SAPy functionalities in SAPy-MMS was determined to
be 10.9 mol% (Table S1, ESI†).
SAPy-MMS shows remarkable chromogenic selectivity for
Fe2+ and Cu2+ ions when tested against various physiologically
important and environmentally toxic metal ions in water (Fig. S5,
ESI†). It is interesting to observe that the metal cations Fe2+ and
Cu2+ caused a remarkable colour change of SAPy-MMS from pale
yellow to dark brown and green, respectively. On the other hand,
the other tested metal ions caused no noticeable colour change,
validating that the prepared SAPy-MMS can be used to quantita-
tively and selectively detect Fe2+ and Cu2+ at lower concentrations
Scheme 1 Synthesis and preparation of the SAPy functionalized mesoporous under basic conditions. For spectrometric detection, metal free
SAPy-MMS in aqueous suspension (0.5 g LÀ1) shows an absorbance
silica nanosensor.
maximum at 410 nm. When the analytes were introduced, the
absorption band of SAPy-MMS shows a noticeable blue shift from
410 nm to 325 nm (85 nm) for Fe2+ and a blue shift from 410 nm to
365 nm (45 nm) for Cu2+ ions (Fig. 2). The absorption shifts were
caused by the formation of SAPy-MMS–Fe2+ and SAPy-MMS–Cu2+
complexes. In addition, the SAPy-MMS–Fe2+ and SAPy-MMS–Cu2+
complex formation caused an internal charge transfer (ICT), which
can be observed as a wavelength shift or colour change.19 Addition
of Fe2+ and Cu2+ produced prominent absorbance enhancements
at A325 nm/A410 nm and A365 nm/A410 nm, respectively.
The effect of pH on SAPy-MMS was determined to investigate
Fig. 1 (A) Nitrogen adsorption–desorption isotherm and pore size distribution
the viable operating range. Fig. 3A(a, b) and B(a, b) show the
(inset) and (B) transmission electron microscopic image of the SAPy-MMS
absorbance of free SAPy-MMS and SAPy-MMS with the addition
nanosensor.
of Fe2+ and Cu2+, respectively, at various pH values. There was
no deviation in the absorption characteristics of free SAPy-MMS
and (210) peak intensities become increased after the surfactant within the pH range of 1 to 14. When Fe2+ or Cu2+ was added, the
extraction process, as expected. The reflection peaks of SAPy-MMS absorbance intensity is enhanced throughout the entire pH range,
after solvent extraction suggest the formation of an ordered
mesoporous silica network. The TEM image of SAPy-MMS shows
also well-ordered, hexagonally arranged mesochannels, as seen in
Fig. 1(b), corroborating that the prepared sample possesses an
ordered mesostructure. Fig. S3(a) (ESI†) shows the SEM image of
SAPy-MMS. The SEM image reveals that some of the particles have
been aggregated or grown together. This most likely happened
during the drying step. As shown in Fig. S3(b) (ESI†) the particle
size was in the range of approximately 400 nm–1.1 mm.
The formation of a covalent bond between the silylated SAPy
Fig. 2 UV-vis absorption spectra of SAPy-MMS with different concentrations of
(A) Fe2+ ions at pH 8 and (B) Cu2+ ions at pH 12.
precursor and tetraethyl orthosilicate (TEOS) was confirmed by
the solid state 29Si MAS NMR spectroscopy (Fig. S4A, ESI†). The
Q signals appeared at À109.1, À100.4, and À90.8 ppm for Q4
[(SiO)4Si], Q3 [(SiO)3Si(OH)], and Q2 [(SiO)2Si(OH)2] structures, respec-
tively; the peaks at À64.0 ppm and À56.3 ppm were assigned to T3
[RSi(OSi)3] and T2 [RSi(OSi)2(OH)] sites, respectively,17 indicating the
high degree of condensation of organosilane functional groups in
the silica pore walls. In the 13C CP MAS NMR spectrum (Fig. S4B,
ESI†), the alkyl carbon resonance signals appeared at 12.4 ppm,
22.6 ppm, and 36.2 ppm for the respective Si–CH2, CH2–CH2–CH2,
and CH2–O groups, indicating the presence of silylated functional
moieties. An intense resonance signal appeared at 177.5 ppm for
the aromatic carbon atoms of benzene and pyridine. Furthermore,
the resonance peaks at 60.4 ppm, 117.8 ppm and 237.0 ppm can
Fig. 3 Effect of pH on absorbance at 325 nm for (A) Fe2+ and 365 nm for (B)
Cu2+, respectively, in UV-vis spectra of (a) free SAPy-MMS and (b) SAPy-MMS–Fe2+
and SAPy-MMS–Cu2+, respectively (6.25 Â 10À6 M).
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cations at concentrations as low as 50 ppb. Thus, SAPy-MMS shows
very favourable properties for its use as an optical solid sensor
with the advantages of simple fabrication, high selectivity,
short response time, regeneration, easy handling, low detection
limit, and low production and operation costs. This method
offers the ability to screen for Fe2+ and Cu2+ in competitive
media by UV-visible spectroscopy or bare human sight. This
approach may aid in the design and development of a new
generation of solid-based optical sensors.
We thank the National Research Foundation of Korea (NRF)
Grant funded by the Ministry of Science, ICT & Future Planning,
Korea (Acceleration Research Program (No. 2013003956), Pioneer
Research Center Program (No. 2013008174/2013008201)).
Fig. 4 Absorption of (a) Fe2+ at 325 nm and (b) Cu2+ at 365 nm upon the
addition of SAPy-MMS to Fe2+ and Cu2+ over the selected competitive metal ions.
attributed to the deprotonation of amide NH groups and pyridine.
In the UV-vis absorption spectra, the relative absorbance maxima
were observed at pH 7–8 for Fe2+ and pH 12 for Cu2+ ions, indicating
the formation of the respective SAPy-MMS–Fe2+ and SAPy-MMS–
Cu2+ complexes.20
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