of UV-light-triggered cellular biochemical changes [124]. Another example of an
advanced application of label-free SERS imaging is a work of Huang et al. [125] on
tracking intracellular drug release. For this purpose, graphene oxide nanoplatforms
were loaded with DOX and AgNPs. SERS spectra of DOX were used to monitor
the real-time process of drug release inside living cells [125].
An indirect SERS probing of cells relies on decoration of NPs by a Raman
reporter exhibiting a specific Raman signature and/or is sensitive to particular
chemical changes/reactions [119]. This approach has been extensively developed in
response to bioanalytical needs to obtain selective and ultrasensitive quantitative
analysis of target molecules present in complex chemical environment. SERS
biosensors can be additionally conjugated with antibodies, aptamers, or peptides as
recognition elements [47, 119]. Such SERS tags have labelling functions similar to
fluorophores but overcome limitations of fluorescence microscopy due to their high
photostability and multiplexing quantification [119, 126].
The indirect SERS technology has found a great application in monitoring of
intracellular pH and redox potential changes. Local pH of intracellular compartments can be measured by using a Raman reporter present on the surface of NPs
that exhibits a strong SERS signal and is sensitive to the pH response. The probe
molecules undergo deprotonation/protonation due to pH variation, resulting in
changes in SERS spectra [127]. Such pH-sensitive Raman reporters are
4-mercaptobenzoic acid (4-MBA) [128–131], 4-ethynylpyridine [132], and
aminotiophenols [130, 133]. For example, intensities of SERS bands assigned to the
deprotonated and protonated 4-MBA carboxylic group at *1720 and *1420 cm
–
1 , respectively, correspond to changes of intracellular pH from 5 to 9 [128]. A great
importance for the determination of subcellular events is also employing SERS tags
for tracking targeted delivery of pH-sensitive drugs [134, 135]. pH-controllable
drug carriers proposed by Chen et al. [134] are bifunctional. They provide
pH-sensitive delivery of the DOX-AuNPs complex and monitor the release of DOX
inside living cells [134].
Chemical reactions of Raman reporters with a target analyte have been used in
designing of SERS sensors detecting the intracellular level of carbon monoxide and
nitric oxide [136, 137]. Carbonylation of the palladacycle moiety was employed to
trap CO molecules [137], whereas monitoring of the NO concentration was possible
due to reaction of NO with a SERS probe producing a benzotriazole derivative
which exhibits in turn an intense Raman band of the –N=N– vibration at 1440 cm
–1
[136].
An effective and selective delivery of anticancer drugs to cancer cells was
proposed by introducing to the design of SERS biosensors conjugation with an
antibody against a target molecule. Song et al. [135] employed SERS probes
conjugated with human epidermal growth factor receptor 2 (HER2) antibody and
observed simultaneously and specifically drug delivery to the SKBR-3 cancer cells
[135]. This approach has been employed in the fabrication of SERS tags for
detection of inflammatory molecules in endothelial cells. Gold nanorods decorated
178
K. Czamara et al.
advanced application of label-free SERS imaging is a work of Huang et al. [125] on
tracking intracellular drug release. For this purpose, graphene oxide nanoplatforms
were loaded with DOX and AgNPs. SERS spectra of DOX were used to monitor
the real-time process of drug release inside living cells [125].
An indirect SERS probing of cells relies on decoration of NPs by a Raman
reporter exhibiting a specific Raman signature and/or is sensitive to particular
chemical changes/reactions [119]. This approach has been extensively developed in
response to bioanalytical needs to obtain selective and ultrasensitive quantitative
analysis of target molecules present in complex chemical environment. SERS
biosensors can be additionally conjugated with antibodies, aptamers, or peptides as
recognition elements [47, 119]. Such SERS tags have labelling functions similar to
fluorophores but overcome limitations of fluorescence microscopy due to their high
photostability and multiplexing quantification [119, 126].
The indirect SERS technology has found a great application in monitoring of
intracellular pH and redox potential changes. Local pH of intracellular compartments can be measured by using a Raman reporter present on the surface of NPs
that exhibits a strong SERS signal and is sensitive to the pH response. The probe
molecules undergo deprotonation/protonation due to pH variation, resulting in
changes in SERS spectra [127]. Such pH-sensitive Raman reporters are
4-mercaptobenzoic acid (4-MBA) [128–131], 4-ethynylpyridine [132], and
aminotiophenols [130, 133]. For example, intensities of SERS bands assigned to the
deprotonated and protonated 4-MBA carboxylic group at *1720 and *1420 cm
–
1 , respectively, correspond to changes of intracellular pH from 5 to 9 [128]. A great
importance for the determination of subcellular events is also employing SERS tags
for tracking targeted delivery of pH-sensitive drugs [134, 135]. pH-controllable
drug carriers proposed by Chen et al. [134] are bifunctional. They provide
pH-sensitive delivery of the DOX-AuNPs complex and monitor the release of DOX
inside living cells [134].
Chemical reactions of Raman reporters with a target analyte have been used in
designing of SERS sensors detecting the intracellular level of carbon monoxide and
nitric oxide [136, 137]. Carbonylation of the palladacycle moiety was employed to
trap CO molecules [137], whereas monitoring of the NO concentration was possible
due to reaction of NO with a SERS probe producing a benzotriazole derivative
which exhibits in turn an intense Raman band of the –N=N– vibration at 1440 cm
–1
[136].
An effective and selective delivery of anticancer drugs to cancer cells was
proposed by introducing to the design of SERS biosensors conjugation with an
antibody against a target molecule. Song et al. [135] employed SERS probes
conjugated with human epidermal growth factor receptor 2 (HER2) antibody and
observed simultaneously and specifically drug delivery to the SKBR-3 cancer cells
[135]. This approach has been employed in the fabrication of SERS tags for
detection of inflammatory molecules in endothelial cells. Gold nanorods decorated
178
K. Czamara et al.