3.1. Raman spectra of serum samples
In Fig. 2 shows the Raman spectra of serum samples from 75 healthy individuals. It can be seen from the spectra that the peak positions of the Raman spectra and the Raman spectrum are the same, and the intensity of each Raman peak changes slightly. Considering that the experimental conditions cannot be exactly the same during the test, for example, the laser power of the sample fluctuates slightly due to inconsistent focusing each time, which affects the intensity of the detection signal and moves the spectrum line up and down.
Figure 3 shows the Raman spectra of serum samples from 69 patients with liver cancer. The spectra showed that the Raman spectra of serum samples from patients with liver cancer had the same peak position. The Raman peak intensity of serum from patients with liver cancer was significantly different from the characteristic peak intensity of serum raman spectra from normal subjects. In order to compare the serum Raman spectra of patients with liver cancer and normal subjects, we averaged the spectra shown in Fig. 4(a).
It can be seen from Fig. 4(a) that the Raman characteristic peaks of liver cancer serum and normal serum mainly occurred in the range of 600–1653cm−1. The main Raman peaks are caused by serum proteins, amino acids, lipids, sugars, carbohydrates and other substances, which occurred in the ranges of 745 cm−1, 1003cm−1, 1127cm−1, 1156cm−1, 1301cm−1, 1337cm−1, 1447cm−1, 1519cm−1 and 1653cm−1. The peak assignments corresponding to their Raman spectra are shown in Table 2. To find the difference between the serum Raman spectra of liver cancer patients and healthy individuals, differential spectra were found by subtraction as shown in Figure 4(b). In liver cancer patients all component contents were significantly reduced compared with those in healthy individuals. The three peaks with the largest difference are 1003, 1156, and 1519 cm−1, which are due to phenylalanine, protein, carotene, carotenoids and porphyrin content variation[28]. Two high intensity Raman peaks at 1156 and 1519 cm−1 due to the resonance raman effect belonging to β-carotene are strongly enhanced under excitation at 532 nm[28, 29]. The decrease in β-carotene in the diseased serum samples is consistent with previous research[29]. In addition, some weak difference Raman peaks appear at 962, 1127, 1297, 1335, 1447, 1584, and 1653cm−1 can also be found. The weak differential peaks at 962 cm−1 belong to ribose C-O stretching of ribose[22], 1127 cm−1 (C-N stretching Protein), 1297 cm−1 (CH2 deformation Fatty acids), 1335 cm−1 (CH3 CH2 wagging, collagen (protein assignment), nucleic acid), 1447 cm−1 (CH2 CH3 bending mode, CH2 deformation of proteins & lipids), 1584 cm−1 (C=C bending mode of phenylalanine)[10], 1653 cm−1 (Carbonyl stretch (C=O), C=C stretch Protein amide I absorption)[39, 40]. Patients with malignant tumors are mostly in a high metabolic state, protein synthesis and catabolism in the body are increased, and the metabolites produced and various material components in the blood are also changed. Amino acids are involved in protein synthesis and catabolite, whose composition and concentration can reflect the metabolic state. Hyperproliferation of tumor cells causes changes in protein, amino acids and other components in body fluid. Rapid growth and unlimited proliferation of cancer cells require a large amount of nutritional substrates, especially amino acids, to be consumed, which will inevitably lead to changes in the amino acid metabolic database of cancer tissue.
Table 2
The spectral peaks and their assignments.
Peaks(cm−1) | Vibrational mode | Major assignment |
745 | Symmetrical skeleton stre-tching, T(ring breathing mode of DNA/RNA bases)[30, 10] | Protein, tryptophan[7], Thymine[27] |
1003 | C-C skeletal | Phenylalanine[30, 10] |
1127 1156 1230 1301 1447 1519 1653 1671 | C-N stretching[31] C-C, C-N stretching In-plane vibrations of the conjugated=C-C=[36], β-carotene accumulation (C=C stretch mode)[37] Antisymmetric stretching vibration[10, 35] C-H vibration, CH2 twisting CH2,CH3 bending mode,CH2 deformation C=C stretch mode C-C & conjugated C=C band stretch Carbonyl stretch (C=O), C=C stretch C=C stretching vibrations, | Protein[30], Protein[30], carotenoids, most likely a cellular pigment[32–34], Glycogen[35] Phosphate[10, 35] Triglycerides (fatty acids)[37], Assign from Parker (lipid)[38], proteins & lipids[10] porphyrin[10], Carotenoid, Carotene[10] Protein amide I absorption[39, 40], Amide I(anti-parallel β-sheet), Cholesterol & its esters, lipids, fatty acids [10, 37] |
3.2. Fluorescence Spectra analysis
Endogenous fluorescent substances are present in the serum, such as proteins, porphyrins, carotenoids, and riboflavin, which can produce fluorescence after excitation by a certain wavelength of light[41–45]. From Fig. 4, we can find that the Raman spectral fluorescence background of liver cancer patients is relatively strong, so we performed fluorescence spectroscopic analysis of the serum of healthy individuals and liver cancer patients. During the experiment, 50 microliters of serum samples were added to 2 ml of saline, diluted and poured into quartz fluorescent colorimetric dishes, put in a fluorescent spectrophotometer (Edinburgh Instruments, FS5 type, UK, with a 150 W xenon lamp as the excitation source, scanning speed of 60nm/min) to obtain physiological saline (background spectroscopy), liver cancer and healthy individual serum fluorescence spectra. The results are shown in Fig. 5. From Fig. 5A, we can see that 462 nm belongs to the fluorescence characteristic peak of physiological saline, with porphyrin luminescence mainly present in the 600-700 nm spectral region[42]. In the spectral region where the largest difference in peak intensity between healthy individuals and liver cancer patients, the molecule playing the main luminescence role is protein[42–46]. Proteins are formed by a peptide chain composed of multiple amino acids repeatedly folding in space, where the amino acids capable of fluorescing are tryptophan, tyrosine, and phenylalanine[43–44]. The growth and division of cancer cells will not be regulated by genes, and their uptake of amino acids is too fast, which disturbs amino acid metabolism in cancer patients, and eventually leads to changes in the content of amino acids in serum[41, 45]. Compared with healthy individuals, liver cancer patients have a reduced ability to degrade aromatic amino acids, and the contents of tryptophan, tyrosine and phenylalanine in serum are significantly increased, with increased concentrations of these three amino acids, leading to enhanced hydrogen bonding energy between light emitting molecules[46–48]. Fluorescence spectra from healthy individual serum and liver cancer patients were used for baseline calibration, and multipeak Gaussian fitting was performed on liver cancer serum (Fig. 5B). We found that the fitted three peaks in the serum of liver cancer patients, 490 nm, 513 nm and 544 nm compared to the three peaks of healthy individuals, 490 nm, 512nm and 580 nm were significantly different in peak position and peak strength. In particular, the peak of serum 544 nm in liver cancer patients was blue-shifted by approximately 36 nm compared with that of healthy individuals 580 nm. This may be due to impaired tissue and organ function in patients with malignant tumors, disrupting amino acid metabolism[45–47]. The content of luminescent amino acids in the free state is increased, and the concentration of amino acids that can emit fluorescence increases, resulting in enhanced hydrogen bond energy and elongation of the two interatomic chemical bonds that form hydrogen bonds[45–47].
3.3. PCA analysis
PCA analysis was performed on the second derivative Raman spectra in range of 1100–1200 cm−1 (Fig. 6). Fig. 6a shows that the serum of patients with liver cancer was well separated from the serum samples of healthy individuals. The first three PCs explained 91% of the total variance, with 53% for PC1, 29% for PC2, and 9% for PC3. The loading plot of PCA is used to identify the peaks that make a high contribution to the differentiated samples. As show in Fig. 6b, PC1 and PC2 mainly contributed greatly near 1127cm−1 and 1156 cm−1, which are related to proteins[28] and carotenoids[34–36].
3.4. PLS-DA results
Performed of PLS-DA analysis were make of calibration set (patients with liver cancer provide 52 serum samples and healthy individuals provide 56 serum samples) and validation set (which patients with liver cancer provide 17 serum samples and healthy individuals gave 19 serum samples) according to the ratio of 3:1 for model work in range of 1100–1200 cm−1 (Fig. 7). From Fig. 7a, we can see that the serum samples are distributed into two clusters. The red cluster is mainly composed of serum samples from patients with liver cancer, and the blue cluster is mainly composed of the serum samples that healthy individuals gave. Fig. 7b shows the loading plot of Factor-1 and Factor-2 for identifying the peaks with high weights in classifying samples. There are positively weighted peaks at approximately 1158 cm−1 and passively weighted peaks at approximately 1154 cm−1. The peak of this region belongs to the Raman peaks of proteins and carotenoids, thus showing that protein and carotenoid changes during liver cancer carcinogenesis dominate in this classification model.
Figure 8 shows the prediction results of PLS-DA in the range of 1100–1200 cm−1. Where predicted Y values greater than zero were considered liver cancer, and less than one was considered healthy. The results showed that the predicted Y values of 17 serum samples from patients with liver cancer and 19 serum samples from healthy individuals were consistent with the actual situation. The effect is very good, and the classification accuracy is 100%