functionalized with streptavidin. When the UCNH is dispersed in a solution with the
target DNA, the capture DNA linked to biotin and the reporter DNA linked to a dye
(a tetramethylrhodamine), the specific interaction between the DNA strands produces a quenching in the 520–540 nm Er emission band, even at low concentrations.
The ratiometric sensor showed linear response from 10 to 50 nM target DNA when
comparing the 540 nm emission band with either the dye emission at 580 nm or the
Er emission at 650 nm.
Our group also developed a ratiometric case (iii) LRET-based sensor. It consisted
of a β-NaYF 4 :Yb,Tm UCNP functionalized with fluorescein, a pH probe (Fig. 3,
Chart 1, Table 1 entry 21), which allowed for pH sensing. The fluorescein
pH-dependent absorption affects the 475 nm Tm emission band, while the Tm
band at 800 nm remained unaffected. The ratiometric sensor was built referencing
the 800 nm emission to that at 475 nm and proved to be linear in the pH range 3–5
(Fig. 5) [54].
Taking into account that many UCNHs have been developed as potential intracellular pH sensors, we have devoted special consideration to the pH sensing topic in
the following section. Last but not least, other biomolecules and ions sensors will be
briefly discussed afterwards.
3.3 pH Sensing
pH plays a crucial role in cellular behaviour, being involved in several cellular
processes such as the metabolism and cell cycle [49, 63, 66]. Depending on the cell
compartment, differences in the pH may be observed. Moreover, an abnormal acidic
pH may symbolize dysfunctions of cells and diseases [66]. Therefore, a lot of efforts
have been made in order to develop efficient pH optical sensors. In this way, many
UCNP-based nanoplatforms have been developed since 2009. Table 1 displays some
pH sensors organized according to their mechanism (i–iii). Some publications have
been selected and discussed below.
Wang et al. developed a case (i) inner-filter pH sensor by loading pH dyes, such
as bromothymol blue and rhodamine B (Fig. 3, Chart 1, Table 1 entry 10), into
bovine serum albumin (BSA), which was attached to the surface of a core-shell
NaYF 4 :Yb,Er@silica UCNP. By varying the pH of the media, the absorption of the
dye and the intensity of the Er emission band at 540 nm vary. The ratio between Er
emission bands at 540 and 650 nm showed a good linearity in the 6–8 and 5–6.5 pH
ranges for bromothymol blue and rhodamine B, respectively.
Giri et al. [51] recently reported a case (ii) dopamine sensor which could be also
used for pH sensing. The system consists of a core-shell β-NaYF 4 :Yb,Tm@NaYbF 4
UCNPs capped with a mesoporous silica shell to facilitate the physical loading of
dopamine molecules (Table 1 entry 20). At neutral and basic pHs, dopamine
undergoes autoxidation to the quinone form, which has an absorption band at
450 nm, and quenches the Tm emission band at the same wavelength (Fig. 6). The
sensor shows good linearity in the 5–8 pH range.
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
381
target DNA, the capture DNA linked to biotin and the reporter DNA linked to a dye
(a tetramethylrhodamine), the specific interaction between the DNA strands produces a quenching in the 520–540 nm Er emission band, even at low concentrations.
The ratiometric sensor showed linear response from 10 to 50 nM target DNA when
comparing the 540 nm emission band with either the dye emission at 580 nm or the
Er emission at 650 nm.
Our group also developed a ratiometric case (iii) LRET-based sensor. It consisted
of a β-NaYF 4 :Yb,Tm UCNP functionalized with fluorescein, a pH probe (Fig. 3,
Chart 1, Table 1 entry 21), which allowed for pH sensing. The fluorescein
pH-dependent absorption affects the 475 nm Tm emission band, while the Tm
band at 800 nm remained unaffected. The ratiometric sensor was built referencing
the 800 nm emission to that at 475 nm and proved to be linear in the pH range 3–5
(Fig. 5) [54].
Taking into account that many UCNHs have been developed as potential intracellular pH sensors, we have devoted special consideration to the pH sensing topic in
the following section. Last but not least, other biomolecules and ions sensors will be
briefly discussed afterwards.
3.3 pH Sensing
pH plays a crucial role in cellular behaviour, being involved in several cellular
processes such as the metabolism and cell cycle [49, 63, 66]. Depending on the cell
compartment, differences in the pH may be observed. Moreover, an abnormal acidic
pH may symbolize dysfunctions of cells and diseases [66]. Therefore, a lot of efforts
have been made in order to develop efficient pH optical sensors. In this way, many
UCNP-based nanoplatforms have been developed since 2009. Table 1 displays some
pH sensors organized according to their mechanism (i–iii). Some publications have
been selected and discussed below.
Wang et al. developed a case (i) inner-filter pH sensor by loading pH dyes, such
as bromothymol blue and rhodamine B (Fig. 3, Chart 1, Table 1 entry 10), into
bovine serum albumin (BSA), which was attached to the surface of a core-shell
NaYF 4 :Yb,Er@silica UCNP. By varying the pH of the media, the absorption of the
dye and the intensity of the Er emission band at 540 nm vary. The ratio between Er
emission bands at 540 and 650 nm showed a good linearity in the 6–8 and 5–6.5 pH
ranges for bromothymol blue and rhodamine B, respectively.
Giri et al. [51] recently reported a case (ii) dopamine sensor which could be also
used for pH sensing. The system consists of a core-shell β-NaYF 4 :Yb,Tm@NaYbF 4
UCNPs capped with a mesoporous silica shell to facilitate the physical loading of
dopamine molecules (Table 1 entry 20). At neutral and basic pHs, dopamine
undergoes autoxidation to the quinone form, which has an absorption band at
450 nm, and quenches the Tm emission band at the same wavelength (Fig. 6). The
sensor shows good linearity in the 5–8 pH range.
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
381